diff --git a/CODE-AUDIT.md b/CODE-AUDIT.md
index adf1ce9..01c7d4a 100644
--- a/CODE-AUDIT.md
+++ b/CODE-AUDIT.md
@@ -33,6 +33,24 @@ fmtBytes/fmtEta/parseProgress unit-tested including NA paths), L38 (webm thumbna
 test), L39 (CROP_SQUARE_PPA structural test), L40 (looksLikeUrl/looksLikeSingleVideo extracted
 to lib/urlHelpers.ts; test imports pure module directly). All verified: typecheck + 242 tests + eslint + prettier green.
 
+**Session 2026-06-30 pass 3 (reliability — B/R/L races & silent failures):** L140 + L148 (the `active`-map
+teardown race — `cancelDownload`/`pauseDownload` release the concurrency slot synchronously and an
+identity-guarded `releaseActive` + per-spawn cookie jar make same-id retry/resume safe), L141 (renderer
+history capped at 500 + url de-dup to mirror main), R6 (`writeJsonAtomic` reports + logs write failures and
+keeps the data dirty for retry instead of an empty catch — log half; toast → CC8), R7 (`encryptSecret` warns
+before the plaintext fallback). All verified: typecheck + 243 tests + eslint green; the 5 touched files are
+prettier-clean. *Deferred this pass (need UI/live-app or larger design): B2/B6 (cancel buttons + abort
+wiring), R4 (can't safely identify a download's `.part` files at cancel time — needs the yt-dlp "Destination"
+path tracked), L139 (needs a spawn↔auto-update interlock), L142 (multi-site IPC-return reconciliation,
+generalises M34), L157 (cross-store cancel-on-remove, ties to C2), R9 (clock-skew — "note for awareness").*
+
+**Session 2026-06-30 pass 4 (Critical — store architecture):** **C1** (single source of truth for the
+preview mock + Settings defaults — `DEFAULT_SETTINGS` in `@shared`, one typed `mockApi.ts`, centralized
+`isPreview`) and **C2** (broke the `downloads ↔ sources` circular import via a pure typed event bus,
+`store/coordinator.ts` — neither store imports the other now). All verified: typecheck + 243 tests +
+eslint green; the touched files are prettier-clean (3 pre-existing format violations in
+DownloadOptionsForm/LibraryView/SettingsView are untouched and out of scope).
+
 **Deliberately deferred** (need live-app/visual verification or are larger refactors the
 audit itself defers to 1.x): the wire-or-cut features (M5/M6/UX1), the god-file/store
 refactors (C1, C2, H1), the SIMP* helpers + shared-UI-token work, the CC* consolidations,
@@ -176,15 +194,30 @@ token system + shared primitives (UI/SIMP — high value, larger effort) · i18n
 
 ## Critical
 
-- [ ] **C1 — Single source of truth for the IPC mock + Settings defaults.** `const PREVIEW`
+- [x] **C1 — Single source of truth for the IPC mock + Settings defaults.** `const PREVIEW`
   redeclared in 8 files; `main.tsx` reimplements the entire `Api` (~180 lines); the full
   `Settings` object is hand-maintained in 3 places (`main/settings.ts` `DEFAULTS`,
   `renderer/store/settings.ts` `FALLBACK`, `main.tsx` `MOCK_SETTINGS`). Add `DEFAULT_SETTINGS`
   to `shared/ipc.ts`, extract the preview mock into one `mockApi.ts` typed as `Api`, centralize
-  `isPreview`.
-- [ ] **C2 — Break the `downloads ↔ sources` store circular dependency.** `downloads.ts`
+  `isPreview`. *Fixed: `DEFAULT_SETTINGS` is now the canonical default in
+  [shared/ipc.ts](src/shared/ipc.ts); main's `DEFAULTS`, the renderer `FALLBACK`
+  ([store/settings.ts](src/renderer/src/store/settings.ts)), and the preview `MOCK_SETTINGS`
+  all derive from it (preview-only field tweaks layered on top) instead of three hand-kept
+  copies. The whole browser mock moved into one [mockApi.ts](src/renderer/src/mockApi.ts) typed
+  `Window['api']` (so a missing/mistyped method is a compile error), and `main.tsx` shrank to
+  `window.api = mockApi`. The `PREVIEW` check is single-sourced in
+  [isPreview.ts](src/renderer/src/isPreview.ts) and imported (as `PREVIEW`) by all 8 sites.*
+- [x] **C2 — Break the `downloads ↔ sources` store circular dependency.** `downloads.ts`
   imports `useSources`; `sources.ts` imports `useDownloads`. Works only via lazy `.getState()`.
-  Introduce a coordinator/event bus that owns cross-store reactions.
+  Introduce a coordinator/event bus that owns cross-store reactions. *Fixed: a tiny typed event
+  bus [store/coordinator.ts](src/renderer/src/store/coordinator.ts) (no runtime deps — the
+  `AddEntry` import is type-only) now owns the two cross-store reactions. `sources` emits
+  `enqueueDownloads` (consumed by `downloads.addMany`); `downloads` emits `downloadCompleted`
+  (consumed by `sources.markDownloaded`). Each store imports the bus, not the other, so the
+  cycle is gone. Because `downloads` no longer pulls in `sources`, `App` now eagerly imports the
+  `sources` store for side-effects (`import './store/sources'`) so its startup load + scheduled-
+  `--sync` kickoff and the `downloadCompleted` subscription still run at launch — robust even
+  once the views are lazy-loaded (PERF8).*
 
 ## High
 
@@ -649,10 +682,19 @@ token system + shared primitives (UI/SIMP — high value, larger effort) · i18n
 - [ ] **L139 — Auto-update vs concurrent spawn race.** `runStartupYtdlpAutoUpdate` can overwrite
   `yt-dlp.exe` on launch while a deep-link/queued download spawns the same binary — a Windows
   file-in-use/corrupt-spawn window.
-- [ ] **L140 — Retry-during-teardown race.** `retry` reuses the same id; if the previous process's
+- [x] **L140 — Retry-during-teardown race.** `retry` reuses the same id; if the previous process's
   `close` handler hasn't removed it from `active` yet, `startDownload` returns "already running" → markError.
-- [ ] **L141 — Renderer history grows unbounded in memory.** `useHistory.add` prepends without a cap
+  *Fixed in [download.ts](src/main/download.ts): `cancelDownload`/`pauseDownload` now drop the item from
+  `active` synchronously (so a retry/resume's same-id spawn is never rejected by the stale entry), and the
+  doomed child's `close`/`error`/watchdog release the slot through a new `releaseActive(id, rec)` that only
+  deletes when that rec still owns the id — so a late teardown can't evict the fresh spawn. The transient
+  cookie jar is now keyed per spawn (`-${spawnSeq}`) so the old download's cleanup can't unlink the new one's
+  file. (with L148)*
+- [x] **L141 — Renderer history grows unbounded in memory.** `useHistory.add` prepends without a cap
   while main caps `history.json` at 500; in a long session the in-memory list exceeds the persisted cap (resets on reload).
+  *Fixed in [store/history.ts](src/renderer/src/store/history.ts): `add` now `.slice(0, MAX_ENTRIES)` (500, the
+  same cap as main's `history.ts`) and de-dupes by `url` as well as `id`, so the optimistic in-memory list
+  matches what a reload would show (also aligns with main's M35 url de-dup).*
 - [ ] **L142 — IPC mutation return values are discarded everywhere.** history/templates/sources/settings
   IPC calls return the authoritative (validated/capped/sanitized) state, but every renderer caller does
   optimistic-only updates and ignores it — client and persisted state can silently diverge until reload (generalizes M34).
@@ -668,8 +710,12 @@ token system + shared primitives (UI/SIMP — high value, larger effort) · i18n
   rather than being rejected up front.
 - [x] **L147 — macOS branches in a Windows-only app.** `app.on('activate')` and the
   `process.platform !== 'darwin'` guard in `window-all-closed` are dead on Windows — boilerplate that implies multi-platform support the app doesn't ship.
-- [ ] **L148 — `clearFinished`/remove can free a slot before the killed process exits.** Removing a
+- [x] **L148 — `clearFinished`/remove can free a slot before the killed process exits.** Removing a
   just-canceled item lets `pump()` launch into the "freed" slot while `taskkill` is still tearing down the prior tree — a brief window over the concurrency cap.
+  *Fixed with L140: `cancelDownload`/`pauseDownload` release the `active` slot the instant they issue the kill
+  (rather than on the async `close`), so main's concurrency accounting matches the renderer immediately — the
+  just-promoted next item is neither falsely rejected by the maxConcurrent guard nor run over the cap while the
+  prior tree dies.*
 
 *Round 7 (2026-06-29) — copy, a11y patterns & micro-behavior:*
 
@@ -1344,12 +1390,22 @@ cosmetics. `R` IDs.
 - [x] **R5 — Settings write failure is unhandled.** `setSettings` calls `store.set(...)` with no try/catch;
   on disk-full/read-only `electron-store` throws → the IPC rejects → the renderer's `setSettings().catch(()
   => {})` swallows it while the optimistic UI keeps the change (the disk-full form of M34). **Fix:** catch and report.
-- [ ] **R6 — Silent data loss on any store write failure.** A completed download that can't be recorded
+- [x] **R6 — Silent data loss on any store write failure.** A completed download that can't be recorded
   (disk full) shows "Completed" in-session but is gone on restart, with no error (the write `catch` is empty,
-  M29). **Fix:** detect write failure and warn.
-- [ ] **R7 — `safeStorage` unavailable ⇒ secrets silently stored as plaintext.** `encryptSecret` falls back
+  M29). **Fix:** detect write failure and warn. *Fixed (log half) in [jsonStore.ts](src/main/jsonStore.ts):
+  `writeJsonAtomic` now returns whether the write landed and logs the failure (with the path) instead of an
+  empty catch, and removes the leftover `.tmp`; `createJsonStore`'s `flush` only clears the dirty flag on
+  success, so a failed write stays pending and is retried on the next `write()` or the quit-time
+  `flushAllStores()` rather than dropped. Unit-tested in `test/jsonStore.test.ts`. **Scope note:** like M29
+  this delivers the "no longer silent" half (logged + retried); the user-facing toast needs the leveled file
+  logger / toast sink deferred to **CC8**.*
+- [x] **R7 — `safeStorage` unavailable ⇒ secrets silently stored as plaintext.** `encryptSecret` falls back
   to plaintext when DPAPI/safeStorage isn't available, with no indication — compounds H7 (cookies) and the
-  backup-cleartext issue (M22). **Fix:** warn (or refuse to store) when encryption is unavailable.
+  backup-cleartext issue (M22). **Fix:** warn (or refuse to store) when encryption is unavailable. *Fixed in
+  [settings.ts](src/main/settings.ts): `encryptSecret` now `console.warn`s before returning a non-empty secret
+  as plaintext, so the cleartext fallback is no longer silent. Chose warn-not-refuse to preserve the
+  pre-encryption behaviour (storing still works); DPAPI is effectively always present on Windows, so this
+  should never fire in practice. (refusing/at-rest enforcement remains an option if ever wanted.)*
 - [x] **R8 — Pretty-printed JSON for large stores.** `JSON.stringify(value, null, 2)` inflates size/write
   time for the potentially-20k-item media store (ties R3). **Fix:** compact JSON for the large stores.
 - [ ] **R9 — Clock-skew sensitivity.** `shouldAutoCheckYtdlp` and the scheduled-download promoter compare
diff --git a/src/main/download.ts b/src/main/download.ts
index d1c6ec3..44662ac 100644
--- a/src/main/download.ts
+++ b/src/main/download.ts
@@ -47,6 +47,21 @@ interface ActiveDownload {
 
 const active = new Map<string, ActiveDownload>()
 
+// Per-spawn sequence, so a retry/resume that reuses the item id still gets a
+// unique transient cookie file (below) — otherwise a superseded download's
+// teardown could unlink the new spawn's jar mid-read.
+let spawnSeq = 0
+
+// Remove an item from the active map, but only if THIS rec still owns the slot.
+// cancel/pause release the slot synchronously (so the renderer's just-promoted
+// next item isn't rejected by the maxConcurrent guard while the killed tree is
+// still tearing down); the doomed child's later 'close'/'error' then runs this as
+// a no-op. The identity check matters because a retry/resume reuses the item id,
+// so a stale teardown must not evict the newer same-id spawn. (L140/L148)
+function releaseActive(id: string, rec: ActiveDownload): void {
+  if (active.get(id) === rec) active.delete(id)
+}
+
 // Backstop for B1: if a spawned yt-dlp goes completely silent — no stdout or
 // stderr — for this long, treat it as wedged, kill it, and error the item so the
 // concurrency slot frees instead of leaking forever. Generous on purpose so a
@@ -269,7 +284,9 @@ export function startDownload(wc: WebContents, opts: StartDownloadOptions): Star
   // settles, so the plaintext never lingers at rest.
   let cookiesFile: string | undefined
   if (getSettings().cookieSource === 'login' && hasStoredCookies()) {
-    const tmp = join(tmpdir(), `aerofetch-cookies-${opts.id}.txt`)
+    // Unique per spawn (not just per id): a retry/resume reuses opts.id, and the
+    // superseded download's cleanupCookies() must not unlink the new spawn's jar.
+    const tmp = join(tmpdir(), `aerofetch-cookies-${opts.id}-${++spawnSeq}.txt`)
     if (materializeCookies(tmp)) cookiesFile = tmp
   }
   function cleanupCookies(): void {
@@ -337,11 +354,14 @@ export function startDownload(wc: WebContents, opts: StartDownloadOptions): Star
   function bumpWatchdog(): void {
     clearWatchdog()
     stallTimer = setTimeout(() => {
-      if (settled) return
+      // A cancel/pause races the timer: it already released the slot and the
+      // child's close stays silent, so don't fire a spurious stall error (mirrors
+      // the canceled/paused guards on the close handler below).
+      if (settled || rec.canceled || rec.paused) return
       settled = true
       clearWatchdog()
       cleanupCookies()
-      active.delete(opts.id)
+      releaseActive(opts.id, rec)
       killTree(rec)
       const msg = `Download stalled — no activity for ${Math.round(STALL_TIMEOUT_MS / 60_000)} min. Stopped; retry to resume.`
       send(wc, { type: 'error', id: opts.id, error: msg })
@@ -385,7 +405,7 @@ export function startDownload(wc: WebContents, opts: StartDownloadOptions): Star
     settled = true
     clearWatchdog()
     cleanupCookies()
-    active.delete(opts.id)
+    releaseActive(opts.id, rec)
     // A paused download was killed on purpose — stay silent, like a cancel.
     if (!rec.canceled && !rec.paused) {
       send(wc, { type: 'error', id: opts.id, error: err.message })
@@ -399,7 +419,7 @@ export function startDownload(wc: WebContents, opts: StartDownloadOptions): Star
     settled = true
     clearWatchdog()
     cleanupCookies()
-    active.delete(opts.id)
+    releaseActive(opts.id, rec)
     // Canceled: renderer already showed 'canceled'. Paused: renderer showed
     // 'paused' and keeps the .part for a later resume. Either way, no event.
     if (rec.canceled || rec.paused) return
@@ -438,6 +458,11 @@ export function cancelDownload(id: string): void {
   const rec = active.get(id)
   if (!rec) return
   rec.canceled = true
+  // Free the slot now (not on the async 'close') so the renderer's just-promoted
+  // next item isn't rejected by the maxConcurrent guard — or run briefly over the
+  // cap — while taskkill tears down this tree. The child's later 'close' stays
+  // silent (rec.canceled) and releaseActive() no-ops. (L148)
+  active.delete(id)
   killTree(rec)
 }
 
@@ -452,5 +477,9 @@ export function pauseDownload(id: string): void {
   const rec = active.get(id)
   if (!rec) return
   rec.paused = true
+  // Free the slot now (see cancelDownload / L148). The partial .part stays on disk
+  // for a later resume, which reuses this id — releaseActive()'s identity check
+  // keeps the doomed child's 'close' from evicting that fresh spawn. (L140)
+  active.delete(id)
   killTree(rec)
 }
diff --git a/src/main/history.ts b/src/main/history.ts
index b9fe1ea..954f9da 100644
--- a/src/main/history.ts
+++ b/src/main/history.ts
@@ -1,16 +1,16 @@
-import type { HistoryEntry } from '@shared/ipc'
+import { HISTORY_MAX_ENTRIES, type HistoryEntry } from '@shared/ipc'
 import { isValidHistoryEntry } from './validation'
 import { createJsonStore } from './jsonStore'
 
 // Plain JSON in userData (portable build redirects userData next to the exe).
 // Kept simple per the build plan; can migrate to better-sqlite3 later. Atomic
 // writes, corruption backup, and caching come from the shared jsonStore (R1–R3).
-const MAX_ENTRIES = 500
-
+// The row cap (HISTORY_MAX_ENTRIES) is shared with the renderer's optimistic list.
+//
 // Per-entry validation (isValidHistoryEntry) so a hand-edited or corrupted
 // history.json can't feed the UI (or openPath) entries with the wrong shape —
 // invalid rows are dropped rather than trusted. (audit S5)
-const store = createJsonStore('history.json', isValidHistoryEntry, MAX_ENTRIES)
+const store = createJsonStore('history.json', isValidHistoryEntry, HISTORY_MAX_ENTRIES)
 
 export function listHistory(): HistoryEntry[] {
   return store.read()
diff --git a/src/main/jsonStore.ts b/src/main/jsonStore.ts
index 4fda7a8..a1853a8 100644
--- a/src/main/jsonStore.ts
+++ b/src/main/jsonStore.ts
@@ -1,6 +1,6 @@
 import { app } from 'electron'
 import { join } from 'path'
-import { readFileSync, writeFileSync, renameSync, existsSync, copyFileSync } from 'fs'
+import { readFileSync, writeFileSync, renameSync, existsSync, copyFileSync, unlinkSync } from 'fs'
 
 /**
  * One shared persistence layer for the hand-rolled JSON array stores (history,
@@ -55,16 +55,29 @@ export function readJsonArraySafe<T>(path: string, isValid: (o: unknown) => o is
  * `rename` it over the target (an atomic operation on the same volume), so a
  * crash mid-write leaves either the old file or the new one -- never a truncated
  * one (R1). `pretty` indents for human-readable files; pass false for large
- * machine-only stores to avoid inflating size/write time (R8). Best-effort: a
- * read-only data dir just means nothing is persisted.
+ * machine-only stores to avoid inflating size/write time (R8). Returns whether the
+ * write landed: a failure (disk full, read-only data dir) is logged rather than
+ * swallowed silently, so a record that looked saved but vanished on restart is at
+ * least diagnosable (R6). The leftover temp file is removed on failure so a partial
+ * `.tmp` can't accumulate.
  */
-export function writeJsonAtomic(path: string, value: unknown, pretty = true): void {
+export function writeJsonAtomic(path: string, value: unknown, pretty = true): boolean {
   const tmp = `${path}.tmp`
   try {
     writeFileSync(tmp, pretty ? JSON.stringify(value, null, 2) : JSON.stringify(value))
     renameSync(tmp, path)
-  } catch {
-    /* best-effort; e.g. a read-only data dir */
+    return true
+  } catch (e) {
+    // R6: previously a silent catch — a failed persist (disk full, read-only
+    // profile) dropped the change with no trace. Log it; the user-facing warning
+    // is deferred to the leveled file logger (CC8).
+    console.error(`[AeroFetch] failed to write ${path}:`, e)
+    try {
+      if (existsSync(tmp)) unlinkSync(tmp)
+    } catch {
+      /* best-effort temp cleanup */
+    }
+    return false
   }
 }
 
@@ -110,8 +123,10 @@ export function createJsonStore<T>(
       timer = null
     }
     if (!dirty) return
-    dirty = false
-    writeJsonAtomic(filePath(), cache ?? [], pretty)
+    // Only clear the dirty flag once the write actually lands. A failed flush
+    // (disk full, transient lock) stays dirty so the next write() or the
+    // quit-time flushAllStores() retries it instead of silently dropping it (R6).
+    if (writeJsonAtomic(filePath(), cache ?? [], pretty)) dirty = false
   }
 
   function write(items: T[]): T[] {
diff --git a/src/main/settings.ts b/src/main/settings.ts
index 4522207..13622ab 100644
--- a/src/main/settings.ts
+++ b/src/main/settings.ts
@@ -13,6 +13,7 @@ import {
   VIDEO_QUALITY_OPTIONS,
   AUDIO_QUALITY_OPTIONS,
   DEFAULT_DOWNLOAD_OPTIONS,
+  DEFAULT_SETTINGS,
   isYtdlpUpdateChannel,
   type Settings,
   type DownloadOptions,
@@ -22,51 +23,11 @@ import {
   type VideoCodecPref
 } from '@shared/ipc'
 
-const DEFAULTS: Settings = {
-  // Both blank by default → downloads land in Documents\Video / Documents\Audio
-  // (see getDefaultMediaDir). A non-empty value is an explicit per-kind override.
-  videoDir: '',
-  audioDir: '',
-  defaultKind: 'video',
-  defaultVideoQuality: 'Best available',
-  defaultAudioQuality: 'Best',
-  maxConcurrent: 2,
-  filenameTemplate: '%(title)s.%(ext)s',
-  // Follow the OS theme on first launch (SR1) so a dark-mode Windows user isn't
-  // greeted by a bright white window despite full system-theme support.
-  theme: 'system',
-  accentColor: 'teal',
-  clipboardWatch: true,
-  downloadOptions: DEFAULT_DOWNLOAD_OPTIONS,
-  proxy: '',
-  rateLimit: '',
-  useAria2c: false,
-  cookieSource: 'none',
-  cookiesBrowser: 'chrome',
-  youtubePlayerClient: '',
-  youtubePoToken: '',
-  restrictFilenames: false,
-  downloadArchive: false,
-  // yt-dlp is self-managed: a writable copy under userData, auto-updated on the
-  // configured channel so a stale binary can't silently cause YouTube 403s.
-  autoUpdateYtdlp: true,
-  // Default to stable so a nightly regression doesn't break downloads for
-  // everyone out of the box (SR2). Users who want the latest YouTube fixes
-  // can switch to nightly in Settings → Software.
-  ytdlpChannel: 'stable',
-  ytdlpLastUpdateCheck: 0,
-  customCommandEnabled: false,
-  defaultTemplateId: null,
-  notifyOnComplete: true,
-  // Default off so adding a watched channel doesn't silently fill the user's
-  // disk on first use (SR3). Enable explicitly once they know what it does.
-  autoDownloadNew: false,
-  hasCompletedOnboarding: false,
-  minimizeToTray: false,
-  launchAtStartup: false,
-  sidebarCollapsed: false,
-  updateToken: ''
-}
+// The electron-store defaults are the canonical DEFAULT_SETTINGS from the shared
+// contract — single-sourced so the renderer FALLBACK and the preview mock can't
+// drift from main (C1). The rationale for individual defaults (SR1/SR2/SR3) lives
+// alongside DEFAULT_SETTINGS in shared/ipc.ts.
+const DEFAULTS: Settings = DEFAULT_SETTINGS
 
 /**
  * Sync the OS "run at sign-in" entry with the launchAtStartup setting. Called at
@@ -197,6 +158,11 @@ function encryptSecret(plain: string): string {
   } catch {
     /* fall through — store plaintext, as it was before encryption existed */
   }
+  // R7: OS encryption (DPAPI/keychain) is unavailable, so this credential is about
+  // to be written in cleartext. Warn rather than fall back silently — a leaked
+  // settings.json would then expose the proxy password / API token. (DPAPI is
+  // effectively always present on Windows, so this should never fire in practice.)
+  console.warn('[AeroFetch] OS secret encryption unavailable — storing credential as plaintext.')
   return plain
 }
 
diff --git a/src/renderer/src/App.tsx b/src/renderer/src/App.tsx
index ecc44b1..a25d778 100644
--- a/src/renderer/src/App.tsx
+++ b/src/renderer/src/App.tsx
@@ -12,6 +12,12 @@ import { LiveRegion } from './components/LiveRegion'
 import { getTheme, pageBackground } from './theme'
 import { useSettings } from './store/settings'
 import { useDownloads } from './store/downloads'
+// Eagerly load the sources store for its startup side-effects (load persisted
+// sources + the watched-source / scheduled `--sync` kickoff) and its cross-store
+// subscription (coordinator's downloadCompleted → markDownloaded). Before C2 this
+// was guaranteed by downloads.ts importing it; now that the cycle is broken, App
+// owns the eager load so it survives the views becoming lazy-loaded (PERF8).
+import './store/sources'
 import { summarizeQueue } from './store/queueStats'
 import { useResolvedDark } from './store/systemTheme'
 import { logError } from './reportError'
diff --git a/src/renderer/src/components/LibraryView.tsx b/src/renderer/src/components/LibraryView.tsx
index d96776c..5ef8645 100644
--- a/src/renderer/src/components/LibraryView.tsx
+++ b/src/renderer/src/components/LibraryView.tsx
@@ -28,6 +28,7 @@ import {
   DismissRegular
 } from '@fluentui/react-icons'
 import type { MediaItem, Source, MediaKind } from '@shared/ipc'
+import { isPreview as PREVIEW } from '../isPreview'
 import { useSources } from '../store/sources'
 import { useSettings } from '../store/settings'
 import { useClipboardLink, looksLikeSingleVideo } from '../useClipboardLink'
@@ -43,9 +44,6 @@ import { SegmentedControl } from './ui/SegmentedControl'
 import { useFocusStyles } from './ui/focusRing'
 import { THUMB_XS } from '../thumbSizes'
 
-// True in the standalone browser preview (no Electron preload).
-const PREVIEW = typeof window === 'undefined' || !window.electron
-
 /** Per-item status shown in the library: a live queue status, or pending/downloaded. */
 type ItemStatus = DownloadStatus | 'pending'
 
diff --git a/src/renderer/src/isPreview.ts b/src/renderer/src/isPreview.ts
new file mode 100644
index 0000000..9d1a723
--- /dev/null
+++ b/src/renderer/src/isPreview.ts
@@ -0,0 +1,8 @@
+/**
+ * True in the standalone browser UI preview, where Electron's preload never ran
+ * so `window.electron` (and the real `window.api` IPC bridge) is absent. The
+ * stores and a few views branch on this to drive seed data / fake tickers instead
+ * of calling IPC. Single-sourced here (C1) so the check isn't re-declared in ~8
+ * files — import it as `PREVIEW` where that reads better locally.
+ */
+export const isPreview = typeof window === 'undefined' || !window.electron
diff --git a/src/renderer/src/main.tsx b/src/renderer/src/main.tsx
index 3bd74d3..1f18d6a 100644
--- a/src/renderer/src/main.tsx
+++ b/src/renderer/src/main.tsx
@@ -1,279 +1,16 @@
 import './assets/base.css'
 import React from 'react'
 import ReactDOM from 'react-dom/client'
-import { DEFAULT_DOWNLOAD_OPTIONS, type Settings, type CommandTemplate } from '@shared/ipc'
 import App from './App'
 import { ErrorBoundary } from './components/ErrorBoundary'
+import { mockApi } from './mockApi'
 
 // In the standalone UI preview (browser, no Electron preload) window.api isn't
-// injected. Stub the full surface so the UI renders and stays interactive during
-// design work. The store detects preview mode (no window.electron) and drives a
-// fake progress ticker instead of calling these, so most are inert no-ops.
-// In the real Electron app this branch is skipped — preload provides window.api.
+// injected. Install the mock surface (mockApi.ts) so the UI renders and stays
+// interactive during design work. In the real Electron app this branch is
+// skipped — preload provides the real window.api.
 if (import.meta.env.DEV && !window.api) {
-  const MOCK_SETTINGS: Settings = {
-    videoDir: 'C:\\Users\\you\\Documents\\Video',
-    audioDir: 'C:\\Users\\you\\Documents\\Audio',
-    defaultKind: 'video',
-    defaultVideoQuality: 'Best available',
-    defaultAudioQuality: 'Best',
-    maxConcurrent: 2,
-    filenameTemplate: '%(title)s.%(ext)s',
-    theme: 'system',
-    accentColor: 'teal',
-    clipboardWatch: true,
-    downloadOptions: DEFAULT_DOWNLOAD_OPTIONS,
-    proxy: '',
-    rateLimit: '',
-    useAria2c: false,
-    cookieSource: 'none',
-    cookiesBrowser: 'chrome',
-    youtubePlayerClient: '',
-    youtubePoToken: '',
-    restrictFilenames: false,
-    downloadArchive: false,
-    autoUpdateYtdlp: true,
-    ytdlpChannel: 'nightly',
-    ytdlpLastUpdateCheck: Date.now() - 3_600_000,
-    customCommandEnabled: false,
-    defaultTemplateId: null,
-    notifyOnComplete: true,
-    autoDownloadNew: true,
-    hasCompletedOnboarding: true,
-    minimizeToTray: false,
-    launchAtStartup: false,
-    sidebarCollapsed: false,
-    updateToken: ''
-  }
-  // Stands in for the cookies.txt file's mtime — lets the Cookies card's
-  // sign-in/clear flow be exercised in this browser-only preview.
-  let mockCookiesSavedAt: number | null = null
-  // Stands in for templates.json — lets the Custom commands card's CRUD and
-  // the download bar's template picker be exercised in this browser-only preview.
-  let mockTemplates: CommandTemplate[] = [
-    { id: 'tpl1', name: 'Write thumbnail, no mtime', args: '--write-thumbnail --no-mtime' }
-  ]
-
-  window.api = {
-    getAppVersion: async () => '0.4.0-preview',
-    checkForAppUpdate: async () => {
-      await new Promise((r) => setTimeout(r, 400))
-      return {
-        ok: true,
-        available: true,
-        currentVersion: '0.4.0',
-        latestVersion: '0.5.0',
-        notes:
-          '### What’s new in v0.5.0\n- In-app updater (this screen!)\n- Faster downloads\n- Bug fixes',
-        htmlUrl: 'https://gitea.netbird.zimspace.uk:5938/debont80/AeroFetch/releases',
-        downloadUrl: 'https://gitea.netbird.zimspace.uk:5938/example/AeroFetch-Setup-0.5.0.exe',
-        assetName: 'AeroFetch-Setup-0.5.0.exe'
-      }
-    },
-    downloadAppUpdate: async () => ({
-      ok: false,
-      error: 'Downloading updates is disabled in the browser preview.'
-    }),
-    runAppUpdate: async () => ({ ok: true }),
-    onAppUpdateProgress: () => () => {},
-    getYtdlpVersion: async () => ({ ok: true, version: '2025.06.01 (UI preview mock)' }),
-    getFfmpegVersions: async () => ({
-      ffmpeg: '7.1-full_build (UI preview mock)',
-      ffprobe: '7.1-full_build (UI preview mock)'
-    }),
-    probe: async (url: string) => {
-      await new Promise((r) => setTimeout(r, 700)) // simulate network latency
-      // A 'list'/'playlist' URL exercises the playlist selection UI in preview.
-      if (/list=|playlist/i.test(url)) {
-        return {
-          ok: true,
-          kind: 'playlist',
-          playlist: {
-            title: 'Full Electron Course — All Episodes',
-            uploader: 'DevChannel',
-            count: 5,
-            entries: Array.from({ length: 5 }, (_, i) => ({
-              index: i + 1,
-              id: `vid${i + 1}`,
-              title: `Episode ${i + 1} — ${['Setup', 'Main Process', 'IPC', 'Packaging', 'Release'][i]}`,
-              url: `https://youtube.com/watch?v=vid${i + 1}`,
-              durationLabel: `${10 + i}:0${i}`,
-              uploader: 'DevChannel'
-            }))
-          }
-        }
-      }
-      return {
-        ok: true,
-        kind: 'video',
-        info: {
-          title: 'Building a Desktop App with Electron — Full Course',
-          channel: 'DevChannel',
-          durationLabel: '1:42:08',
-          thumbnail: undefined,
-          formats: [
-            { id: '__best__', label: 'Best available', ext: 'mp4', hasAudio: true },
-            {
-              id: '299',
-              label: '1080p60 · mp4 · 248 MB',
-              height: 1080,
-              ext: 'mp4',
-              filesizeLabel: '248 MB',
-              hasAudio: false
-            },
-            {
-              id: '136',
-              label: '720p · mp4 · 124 MB',
-              height: 720,
-              ext: 'mp4',
-              filesizeLabel: '124 MB',
-              hasAudio: false
-            },
-            {
-              id: '135',
-              label: '480p · mp4 · 72 MB',
-              height: 480,
-              ext: 'mp4',
-              filesizeLabel: '72 MB',
-              hasAudio: false
-            },
-            {
-              id: '134',
-              label: '360p · mp4 · 38 MB',
-              height: 360,
-              ext: 'mp4',
-              filesizeLabel: '38 MB',
-              hasAudio: false
-            }
-          ]
-        }
-      }
-    },
-    startDownload: async () => ({ ok: true }),
-    cancelDownload: async () => {},
-    pauseDownload: async () => {},
-    chooseFolder: async () => null,
-    openPath: async () => '',
-    openUrl: async () => {},
-    showInFolder: async () => {},
-    readClipboard: async () => '',
-    getSettings: async () => MOCK_SETTINGS,
-    setSettings: async (partial) => Object.assign(MOCK_SETTINGS, partial),
-    listHistory: async () => [],
-    addHistory: async () => [],
-    removeHistory: async () => [],
-    removeManyHistory: async () => [],
-    clearHistory: async () => [],
-    cookiesLogin: async () => {
-      await new Promise((r) => setTimeout(r, 600)) // simulate the sign-in window
-      mockCookiesSavedAt = Date.now()
-      return { ok: true, cookieCount: 14 }
-    },
-    cookiesStatus: async () =>
-      mockCookiesSavedAt ? { exists: true, savedAt: mockCookiesSavedAt } : { exists: false },
-    cookiesClear: async () => {
-      mockCookiesSavedAt = null
-    },
-    listTemplates: async () => mockTemplates,
-    saveTemplate: async (template) => {
-      mockTemplates = [template, ...mockTemplates.filter((t) => t.id !== template.id)]
-      return mockTemplates
-    },
-    removeTemplate: async (id) => {
-      mockTemplates = mockTemplates.filter((t) => t.id !== id)
-      return mockTemplates
-    },
-    previewCommand: async (opts) => {
-      await new Promise((r) => setTimeout(r, 300)) // simulate the main-process round-trip
-      const extra = opts.extraArgs ? ` ${opts.extraArgs}` : ''
-      const dir = opts.kind === 'audio' ? MOCK_SETTINGS.audioDir : MOCK_SETTINGS.videoDir
-      return {
-        ok: true,
-        command:
-          `yt-dlp.exe -f "bv*+ba/b" -o "${dir}\\%(title)s.%(ext)s"` + `${extra} -- ${opts.url}`
-      }
-    },
-    updateYtdlp: async (channel) => {
-      await new Promise((r) => setTimeout(r, 600)) // simulate the self-update run
-      return {
-        ok: true,
-        output: `yt-dlp is already up to date (channel: ${channel}, UI preview mock)`
-      }
-    },
-    // No background updater in the browser preview — hand back an inert unsubscribe.
-    onYtdlpAutoUpdateStatus: () => () => {},
-    listErrorLog: async () => [],
-    clearErrorLog: async () => [],
-    exportBackup: async () => ({
-      ok: true,
-      path: 'C:\\Users\\you\\Downloads\\aerofetch-backup.json'
-    }),
-    importBackup: async () => ({ ok: true }),
-    onDownloadEvent: () => () => {},
-    getSystemTheme: async () => ({
-      shouldUseDarkColors: false,
-      shouldUseHighContrastColors: false
-    }),
-    onSystemThemeUpdate: () => () => {},
-    openHighContrastSettings: async () => {},
-    onExternalUrl: () => () => {},
-    // Media-manager sources — a seeded channel so the (Phase H) Library view has
-    // demo data in this browser-only preview.
-    listSources: async () => [
-      {
-        id: 'src-demo',
-        url: 'https://www.youtube.com/@DevChannel',
-        kind: 'channel',
-        title: 'DevChannel',
-        channel: 'DevChannel',
-        addedAt: Date.now() - 86_400_000,
-        lastIndexedAt: Date.now() - 3_600_000,
-        itemCount: 7
-      }
-    ],
-    indexSource: async (url) => {
-      await new Promise((r) => setTimeout(r, 800)) // simulate the index walk
-      return {
-        ok: true,
-        source: {
-          id: 'src-demo',
-          url,
-          kind: 'channel',
-          title: 'DevChannel',
-          channel: 'DevChannel',
-          addedAt: Date.now(),
-          lastIndexedAt: Date.now(),
-          itemCount: 7
-        },
-        itemCount: 7
-      }
-    },
-    reindexSource: async () => ({ ok: true, itemCount: 7, newCount: 0 }),
-    removeSource: async () => [],
-    markSourceItemDownloaded: async () => [],
-    setSourceWatched: async () => [],
-    syncSources: async () => ({ ok: true, newItems: [] }),
-    getScheduledSync: async () => ({ enabled: false }),
-    setScheduledSync: async (enabled: boolean) => ({ enabled }),
-    listSourceItems: async (sourceId) =>
-      Array.from({ length: 7 }, (_, i) => ({
-        id: `${sourceId}:vid${i + 1}`,
-        sourceId,
-        videoId: `vid${i + 1}`,
-        title: `Episode ${i + 1}`,
-        url: `https://youtube.com/watch?v=vid${i + 1}`,
-        playlistTitle: i < 5 ? 'Full Electron Course' : 'Uploads',
-        playlistIndex: i < 5 ? i + 1 : i - 4,
-        durationLabel: `${10 + i}:0${i}`,
-        downloaded: i < 2
-      })),
-    onIndexProgress: () => () => {},
-    runTerminal: async () => ({ ok: true }),
-    cancelTerminal: async () => {},
-    onTerminalOutput: () => () => {},
-    setTaskbarProgress: async () => {},
-    mintPoToken: async () => null
-  }
+  window.api = mockApi
 }
 
 ReactDOM.createRoot(document.getElementById('root') as HTMLElement).render(
diff --git a/src/renderer/src/mockApi.ts b/src/renderer/src/mockApi.ts
new file mode 100644
index 0000000..4440c67
--- /dev/null
+++ b/src/renderer/src/mockApi.ts
@@ -0,0 +1,255 @@
+import { DEFAULT_SETTINGS, type Settings, type CommandTemplate } from '@shared/ipc'
+
+/**
+ * The browser-preview stand-in for the real `window.api` IPC bridge (C1). In the
+ * standalone UI preview (browser, no Electron preload) `window.api` isn't
+ * injected, so main.tsx assigns this mock to keep the UI interactive during
+ * design work. Typed as the real `Api` surface (`Window['api']`) so it can't
+ * silently drift from the preload contract — a missing or mistyped method is a
+ * compile error. Most methods are inert no-ops because the stores detect preview
+ * mode (no window.electron) and drive a fake progress ticker instead.
+ */
+type Api = Window['api']
+
+// The preview settings: the canonical defaults, plus preview-only tweaks —
+// placeholder folders, a nightly channel + a recent check to exercise the
+// updater card, auto-download on, and onboarding already dismissed so design
+// work isn't blocked behind the welcome screen.
+const MOCK_SETTINGS: Settings = {
+  ...DEFAULT_SETTINGS,
+  videoDir: 'C:\\Users\\you\\Documents\\Video',
+  audioDir: 'C:\\Users\\you\\Documents\\Audio',
+  ytdlpChannel: 'nightly',
+  ytdlpLastUpdateCheck: Date.now() - 3_600_000,
+  autoDownloadNew: true,
+  hasCompletedOnboarding: true
+}
+
+// Stands in for the cookies.txt file's mtime — lets the Cookies card's
+// sign-in/clear flow be exercised in this browser-only preview.
+let mockCookiesSavedAt: number | null = null
+// Stands in for templates.json — lets the Custom commands card's CRUD and
+// the download bar's template picker be exercised in this browser-only preview.
+let mockTemplates: CommandTemplate[] = [
+  { id: 'tpl1', name: 'Write thumbnail, no mtime', args: '--write-thumbnail --no-mtime' }
+]
+
+export const mockApi: Api = {
+  getAppVersion: async () => '0.4.0-preview',
+  checkForAppUpdate: async () => {
+    await new Promise((r) => setTimeout(r, 400))
+    return {
+      ok: true,
+      available: true,
+      currentVersion: '0.4.0',
+      latestVersion: '0.5.0',
+      notes:
+        '### What’s new in v0.5.0\n- In-app updater (this screen!)\n- Faster downloads\n- Bug fixes',
+      htmlUrl: 'https://gitea.netbird.zimspace.uk:5938/debont80/AeroFetch/releases',
+      downloadUrl: 'https://gitea.netbird.zimspace.uk:5938/example/AeroFetch-Setup-0.5.0.exe',
+      assetName: 'AeroFetch-Setup-0.5.0.exe'
+    }
+  },
+  downloadAppUpdate: async () => ({
+    ok: false,
+    error: 'Downloading updates is disabled in the browser preview.'
+  }),
+  runAppUpdate: async () => ({ ok: true }),
+  onAppUpdateProgress: () => () => {},
+  getYtdlpVersion: async () => ({ ok: true, version: '2025.06.01 (UI preview mock)' }),
+  getFfmpegVersions: async () => ({
+    ffmpeg: '7.1-full_build (UI preview mock)',
+    ffprobe: '7.1-full_build (UI preview mock)'
+  }),
+  probe: async (url: string) => {
+    await new Promise((r) => setTimeout(r, 700)) // simulate network latency
+    // A 'list'/'playlist' URL exercises the playlist selection UI in preview.
+    if (/list=|playlist/i.test(url)) {
+      return {
+        ok: true,
+        kind: 'playlist',
+        playlist: {
+          title: 'Full Electron Course — All Episodes',
+          uploader: 'DevChannel',
+          count: 5,
+          entries: Array.from({ length: 5 }, (_, i) => ({
+            index: i + 1,
+            id: `vid${i + 1}`,
+            title: `Episode ${i + 1} — ${['Setup', 'Main Process', 'IPC', 'Packaging', 'Release'][i]}`,
+            url: `https://youtube.com/watch?v=vid${i + 1}`,
+            durationLabel: `${10 + i}:0${i}`,
+            uploader: 'DevChannel'
+          }))
+        }
+      }
+    }
+    return {
+      ok: true,
+      kind: 'video',
+      info: {
+        title: 'Building a Desktop App with Electron — Full Course',
+        channel: 'DevChannel',
+        durationLabel: '1:42:08',
+        thumbnail: undefined,
+        formats: [
+          { id: '__best__', label: 'Best available', ext: 'mp4', hasAudio: true },
+          {
+            id: '299',
+            label: '1080p60 · mp4 · 248 MB',
+            height: 1080,
+            ext: 'mp4',
+            filesizeLabel: '248 MB',
+            hasAudio: false
+          },
+          {
+            id: '136',
+            label: '720p · mp4 · 124 MB',
+            height: 720,
+            ext: 'mp4',
+            filesizeLabel: '124 MB',
+            hasAudio: false
+          },
+          {
+            id: '135',
+            label: '480p · mp4 · 72 MB',
+            height: 480,
+            ext: 'mp4',
+            filesizeLabel: '72 MB',
+            hasAudio: false
+          },
+          {
+            id: '134',
+            label: '360p · mp4 · 38 MB',
+            height: 360,
+            ext: 'mp4',
+            filesizeLabel: '38 MB',
+            hasAudio: false
+          }
+        ]
+      }
+    }
+  },
+  startDownload: async () => ({ ok: true }),
+  cancelDownload: async () => {},
+  pauseDownload: async () => {},
+  chooseFolder: async () => null,
+  openPath: async () => '',
+  openUrl: async () => {},
+  showInFolder: async () => {},
+  readClipboard: async () => '',
+  getSettings: async () => MOCK_SETTINGS,
+  setSettings: async (partial) => Object.assign(MOCK_SETTINGS, partial),
+  listHistory: async () => [],
+  addHistory: async () => [],
+  removeHistory: async () => [],
+  removeManyHistory: async () => [],
+  clearHistory: async () => [],
+  cookiesLogin: async () => {
+    await new Promise((r) => setTimeout(r, 600)) // simulate the sign-in window
+    mockCookiesSavedAt = Date.now()
+    return { ok: true, cookieCount: 14 }
+  },
+  cookiesStatus: async () =>
+    mockCookiesSavedAt ? { exists: true, savedAt: mockCookiesSavedAt } : { exists: false },
+  cookiesClear: async () => {
+    mockCookiesSavedAt = null
+  },
+  listTemplates: async () => mockTemplates,
+  saveTemplate: async (template) => {
+    mockTemplates = [template, ...mockTemplates.filter((t) => t.id !== template.id)]
+    return mockTemplates
+  },
+  removeTemplate: async (id) => {
+    mockTemplates = mockTemplates.filter((t) => t.id !== id)
+    return mockTemplates
+  },
+  previewCommand: async (opts) => {
+    await new Promise((r) => setTimeout(r, 300)) // simulate the main-process round-trip
+    const extra = opts.extraArgs ? ` ${opts.extraArgs}` : ''
+    const dir = opts.kind === 'audio' ? MOCK_SETTINGS.audioDir : MOCK_SETTINGS.videoDir
+    return {
+      ok: true,
+      command: `yt-dlp.exe -f "bv*+ba/b" -o "${dir}\\%(title)s.%(ext)s"` + `${extra} -- ${opts.url}`
+    }
+  },
+  updateYtdlp: async (channel) => {
+    await new Promise((r) => setTimeout(r, 600)) // simulate the self-update run
+    return {
+      ok: true,
+      output: `yt-dlp is already up to date (channel: ${channel}, UI preview mock)`
+    }
+  },
+  // No background updater in the browser preview — hand back an inert unsubscribe.
+  onYtdlpAutoUpdateStatus: () => () => {},
+  listErrorLog: async () => [],
+  clearErrorLog: async () => [],
+  exportBackup: async () => ({
+    ok: true,
+    path: 'C:\\Users\\you\\Downloads\\aerofetch-backup.json'
+  }),
+  importBackup: async () => ({ ok: true }),
+  onDownloadEvent: () => () => {},
+  getSystemTheme: async () => ({
+    shouldUseDarkColors: false,
+    shouldUseHighContrastColors: false
+  }),
+  onSystemThemeUpdate: () => () => {},
+  openHighContrastSettings: async () => {},
+  onExternalUrl: () => () => {},
+  // Media-manager sources — a seeded channel so the (Phase H) Library view has
+  // demo data in this browser-only preview.
+  listSources: async () => [
+    {
+      id: 'src-demo',
+      url: 'https://www.youtube.com/@DevChannel',
+      kind: 'channel',
+      title: 'DevChannel',
+      channel: 'DevChannel',
+      addedAt: Date.now() - 86_400_000,
+      lastIndexedAt: Date.now() - 3_600_000,
+      itemCount: 7
+    }
+  ],
+  indexSource: async (url) => {
+    await new Promise((r) => setTimeout(r, 800)) // simulate the index walk
+    return {
+      ok: true,
+      source: {
+        id: 'src-demo',
+        url,
+        kind: 'channel',
+        title: 'DevChannel',
+        channel: 'DevChannel',
+        addedAt: Date.now(),
+        lastIndexedAt: Date.now(),
+        itemCount: 7
+      },
+      itemCount: 7
+    }
+  },
+  reindexSource: async () => ({ ok: true, itemCount: 7, newCount: 0 }),
+  removeSource: async () => [],
+  markSourceItemDownloaded: async () => [],
+  setSourceWatched: async () => [],
+  syncSources: async () => ({ ok: true, newItems: [] }),
+  getScheduledSync: async () => ({ enabled: false }),
+  setScheduledSync: async (enabled: boolean) => ({ enabled }),
+  listSourceItems: async (sourceId) =>
+    Array.from({ length: 7 }, (_, i) => ({
+      id: `${sourceId}:vid${i + 1}`,
+      sourceId,
+      videoId: `vid${i + 1}`,
+      title: `Episode ${i + 1}`,
+      url: `https://youtube.com/watch?v=vid${i + 1}`,
+      playlistTitle: i < 5 ? 'Full Electron Course' : 'Uploads',
+      playlistIndex: i < 5 ? i + 1 : i - 4,
+      durationLabel: `${10 + i}:0${i}`,
+      downloaded: i < 2
+    })),
+  onIndexProgress: () => () => {},
+  runTerminal: async () => ({ ok: true }),
+  cancelTerminal: async () => {},
+  onTerminalOutput: () => () => {},
+  setTaskbarProgress: async () => {},
+  mintPoToken: async () => null
+}
diff --git a/src/renderer/src/store/coordinator.ts b/src/renderer/src/store/coordinator.ts
new file mode 100644
index 0000000..95e78f2
--- /dev/null
+++ b/src/renderer/src/store/coordinator.ts
@@ -0,0 +1,48 @@
+import type { AddEntry } from './downloads'
+
+/**
+ * A tiny typed event bus that owns the cross-store reactions between the
+ * downloads and sources stores (C2). Previously `downloads.ts` imported
+ * `useSources` and `sources.ts` imported `useDownloads`, a circular dependency
+ * that only worked because both sides reached across lazily via `.getState()`.
+ *
+ * Now each store depends on this bus instead of on the other: a store *emits* the
+ * events it produces and *subscribes* to the ones it consumes. The bus itself has
+ * no runtime dependency on either store (the `AddEntry` import is type-only and is
+ * erased at build time), so there is no import cycle.
+ *
+ *   sources  --enqueueDownloads-->  downloads   (queue a channel's media items)
+ *   downloads --downloadCompleted--> sources    (mark the source item downloaded)
+ */
+export interface StoreEvents {
+  /** sources → downloads: enqueue a batch of media items as downloads */
+  enqueueDownloads: AddEntry[]
+  /** downloads → sources: a download tied to a source MediaItem finished */
+  downloadCompleted: { mediaItemId: string; filePath?: string }
+}
+
+type Handler<K extends keyof StoreEvents> = (payload: StoreEvents[K]) => void
+
+// Heterogeneous handler sets keyed by event name. The `never` payload is the
+// standard trick for a per-key event map; the public on/emit signatures keep the
+// payload types sound at every call site.
+const handlers = new Map<keyof StoreEvents, Set<(payload: never) => void>>()
+
+/** Subscribe to a store event. Returns an unsubscribe function. */
+export function on<K extends keyof StoreEvents>(event: K, handler: Handler<K>): () => void {
+  let set = handlers.get(event)
+  if (!set) {
+    set = new Set()
+    handlers.set(event, set)
+  }
+  const erased = handler as (payload: never) => void
+  set.add(erased)
+  return () => set.delete(erased)
+}
+
+/** Dispatch a store event to all current subscribers. */
+export function emit<K extends keyof StoreEvents>(event: K, payload: StoreEvents[K]): void {
+  const set = handlers.get(event)
+  if (!set) return
+  for (const handler of set) (handler as Handler<K>)(payload)
+}
diff --git a/src/renderer/src/store/downloads.ts b/src/renderer/src/store/downloads.ts
index b3cc604..6da1ce9 100644
--- a/src/renderer/src/store/downloads.ts
+++ b/src/renderer/src/store/downloads.ts
@@ -6,9 +6,10 @@ import {
   type CollectionContext,
   type MediaKind
 } from '@shared/ipc'
+import { isPreview as PREVIEW } from '../isPreview'
 import { useSettings } from './settings'
 import { useHistory } from './history'
-import { useSources } from './sources'
+import { emit, on } from './coordinator'
 import { newId } from '../id'
 
 // Single-sourced from the IPC contract (L105) and re-exported so existing
@@ -99,10 +100,6 @@ export interface AddEntry {
   opts?: AddOptions
 }
 
-// True when running in the standalone browser preview (no Electron preload).
-// The real preload injects window.electron; the browser stub does not.
-const PREVIEW = typeof window === 'undefined' || !window.electron
-
 interface DownloadState {
   items: DownloadItem[]
   addFromUrl: (url: string, kind: MediaKind, quality: string, opts?: AddOptions) => void
@@ -274,7 +271,9 @@ function recordCompletion(item: DownloadItem): void {
     })
   }
   if (item.mediaItemId) {
-    useSources.getState().markDownloaded(item.mediaItemId, item.filePath)
+    // Tell the sources store to mark its MediaItem downloaded — via the event bus
+    // rather than a direct import, so downloads and sources don't form a cycle (C2).
+    emit('downloadCompleted', { mediaItemId: item.mediaItemId, filePath: item.filePath })
   }
 }
 
@@ -628,6 +627,11 @@ export const useDownloads = create<DownloadState>((set, get) => {
   }
 })
 
+// Consume the sources store's enqueue requests (C2): when the user downloads a
+// channel's media items, sources emits the batch on the bus and we add it here —
+// no direct import from sources, so the two stores stay acyclic.
+on('enqueueDownloads', (entries) => useDownloads.getState().addMany(entries))
+
 // Promote scheduled ('saved' + a due scheduledFor) items when their time arrives.
 // Session-only: the queue isn't persisted, so a schedule only fires while the app
 // runs (noted in ROADMAP.md). A 15s tick is plenty for minute-granularity times.
diff --git a/src/renderer/src/store/errorlog.ts b/src/renderer/src/store/errorlog.ts
index dda7b77..9e53799 100644
--- a/src/renderer/src/store/errorlog.ts
+++ b/src/renderer/src/store/errorlog.ts
@@ -1,10 +1,8 @@
 import { create } from 'zustand'
 import type { ErrorLogEntry } from '@shared/ipc'
+import { isPreview as PREVIEW } from '../isPreview'
 import { logError } from '../reportError'
 
-// True in the standalone browser preview (no Electron preload).
-const PREVIEW = typeof window === 'undefined' || !window.electron
-
 // Sample entries so the Diagnostics card has content during UI design.
 const seed: ErrorLogEntry[] = PREVIEW
   ? [
diff --git a/src/renderer/src/store/history.ts b/src/renderer/src/store/history.ts
index a123424..c37f4cd 100644
--- a/src/renderer/src/store/history.ts
+++ b/src/renderer/src/store/history.ts
@@ -1,10 +1,8 @@
 import { create } from 'zustand'
-import type { HistoryEntry } from '@shared/ipc'
+import { HISTORY_MAX_ENTRIES, type HistoryEntry } from '@shared/ipc'
+import { isPreview as PREVIEW } from '../isPreview'
 import { logError } from '../reportError'
 
-// True in the standalone browser preview (no Electron preload).
-const PREVIEW = typeof window === 'undefined' || !window.electron
-
 // Mock history so the History tab has content during UI design.
 const seed: HistoryEntry[] = PREVIEW
   ? [
@@ -44,6 +42,10 @@ const seed: HistoryEntry[] = PREVIEW
     ]
   : []
 
+// Cap the optimistic in-memory list at the same shared limit main persists to
+// (HISTORY_MAX_ENTRIES) so a long session can't grow it past what a reload would
+// show (L141). De-duping by url as well as id keeps it consistent with main's add
+// (M35), so the optimistic list matches the reload.
 interface HistoryState {
   entries: HistoryEntry[]
   add: (entry: HistoryEntry) => void
@@ -58,7 +60,12 @@ export const useHistory = create<HistoryState>((set, get) => ({
   entries: seed,
 
   add: (entry) => {
-    set((s) => ({ entries: [entry, ...s.entries.filter((e) => e.id !== entry.id)] }))
+    set((s) => ({
+      entries: [entry, ...s.entries.filter((e) => e.id !== entry.id && e.url !== entry.url)].slice(
+        0,
+        HISTORY_MAX_ENTRIES
+      )
+    }))
     if (!PREVIEW) window.api.addHistory(entry).catch(logError('addHistory'))
   },
 
diff --git a/src/renderer/src/store/settings.ts b/src/renderer/src/store/settings.ts
index f24e4d8..6455e7f 100644
--- a/src/renderer/src/store/settings.ts
+++ b/src/renderer/src/store/settings.ts
@@ -1,48 +1,22 @@
 import { create } from 'zustand'
-import { DEFAULT_DOWNLOAD_OPTIONS, type Settings } from '@shared/ipc'
+import { DEFAULT_SETTINGS, type Settings } from '@shared/ipc'
+import { isPreview as PREVIEW } from '../isPreview'
 import { logError } from '../reportError'
 
-// True in the standalone browser preview (no Electron preload).
-const PREVIEW = typeof window === 'undefined' || !window.electron
-
-const FALLBACK: Settings = {
-  videoDir: PREVIEW ? 'C:\\Users\\you\\Documents\\Video' : '',
-  audioDir: PREVIEW ? 'C:\\Users\\you\\Documents\\Audio' : '',
-  defaultKind: 'video',
-  defaultVideoQuality: 'Best available',
-  defaultAudioQuality: 'Best',
-  maxConcurrent: 2,
-  filenameTemplate: '%(title)s.%(ext)s',
-  // Mirror main's DEFAULTS (SR1): follow the OS theme until real settings load,
-  // so there's no white-on-first-paint flash for a dark-mode user.
-  theme: 'system',
-  accentColor: 'teal',
-  clipboardWatch: true,
-  downloadOptions: DEFAULT_DOWNLOAD_OPTIONS,
-  proxy: '',
-  rateLimit: '',
-  useAria2c: false,
-  cookieSource: 'none',
-  cookiesBrowser: 'chrome',
-  youtubePlayerClient: '',
-  youtubePoToken: '',
-  restrictFilenames: false,
-  downloadArchive: false,
-  autoUpdateYtdlp: true,
-  ytdlpChannel: 'stable',
-  ytdlpLastUpdateCheck: 0,
-  customCommandEnabled: false,
-  defaultTemplateId: null,
-  notifyOnComplete: true,
-  autoDownloadNew: false,
-  // True in preview so design work isn't blocked behind the welcome screen;
-  // a real first launch gets `false` from main/settings.ts's DEFAULTS instead.
-  hasCompletedOnboarding: PREVIEW,
-  minimizeToTray: false,
-  launchAtStartup: false,
-  sidebarCollapsed: false,
-  updateToken: ''
-}
+// The pre-load fallback is the canonical DEFAULT_SETTINGS (single-sourced from the
+// shared contract, C1) — including theme:'system' (SR1) so there's no white-on-
+// first-paint flash for a dark-mode user before the real settings arrive. In the
+// browser preview a couple of fields differ: placeholder folder paths, and
+// onboarding pre-dismissed so design work isn't blocked behind the welcome screen
+// (a real first launch keeps DEFAULT_SETTINGS' false from main).
+const FALLBACK: Settings = PREVIEW
+  ? {
+      ...DEFAULT_SETTINGS,
+      videoDir: 'C:\\Users\\you\\Documents\\Video',
+      audioDir: 'C:\\Users\\you\\Documents\\Audio',
+      hasCompletedOnboarding: true
+    }
+  : DEFAULT_SETTINGS
 
 interface SettingsState extends Settings {
   /** true once persisted settings have loaded (always true in preview) */
diff --git a/src/renderer/src/store/sources.ts b/src/renderer/src/store/sources.ts
index 76d58c0..4c27c3b 100644
--- a/src/renderer/src/store/sources.ts
+++ b/src/renderer/src/store/sources.ts
@@ -1,12 +1,10 @@
 import { create } from 'zustand'
 import type { Source, MediaItem, IndexProgress, MediaKind } from '@shared/ipc'
-import { useDownloads } from './downloads'
+import { isPreview as PREVIEW } from '../isPreview'
 import { useSettings } from './settings'
+import { emit, on } from './coordinator'
 import { logError } from '../reportError'
 
-// True in the standalone browser preview (no Electron preload).
-const PREVIEW = typeof window === 'undefined' || !window.electron
-
 // --- Preview seed data ------------------------------------------------------
 
 function seedItems(sourceId: string, playlist: string, n: number, downloadedUpTo = 0): MediaItem[] {
@@ -185,8 +183,11 @@ export const useSources = create<SourcesState>((set, get) => ({
     // Per-batch override (Library video/audio toggle) falls back to the global default (M27).
     const kind = kindOverride ?? settings.defaultKind
     const quality = kind === 'video' ? settings.defaultVideoQuality : settings.defaultAudioQuality
-    // One batched state update + pump, so queuing a whole channel stays O(n).
-    useDownloads.getState().addMany(
+    // Hand the batch to the downloads store via the event bus (C2) rather than a
+    // direct import. addMany applies it as one batched state update + pump, so
+    // queuing a whole channel stays O(n).
+    emit(
+      'enqueueDownloads',
       items.map((it) => ({
         url: it.url,
         kind,
@@ -268,6 +269,12 @@ export const useSources = create<SourcesState>((set, get) => ({
   }
 }))
 
+// When a download tied to one of our MediaItems finishes, the downloads store
+// emits this on the bus (C2) — mark the item downloaded so the library reflects it.
+on('downloadCompleted', ({ mediaItemId, filePath }) =>
+  useSources.getState().markDownloaded(mediaItemId, filePath)
+)
+
 // Load persisted sources on startup, and subscribe to live indexing progress.
 if (!PREVIEW) {
   window.api
diff --git a/src/renderer/src/store/systemTheme.ts b/src/renderer/src/store/systemTheme.ts
index 12095c9..cbe2fd8 100644
--- a/src/renderer/src/store/systemTheme.ts
+++ b/src/renderer/src/store/systemTheme.ts
@@ -1,10 +1,8 @@
 import { create } from 'zustand'
 import type { SystemThemeInfo } from '@shared/ipc'
+import { isPreview as PREVIEW } from '../isPreview'
 import { useSettings } from './settings'
 
-// True in the standalone browser preview (no Electron preload).
-const PREVIEW = typeof window === 'undefined' || !window.electron
-
 const prefersDark =
   typeof window !== 'undefined' && window.matchMedia
     ? window.matchMedia('(prefers-color-scheme: dark)')
diff --git a/src/renderer/src/store/templates.ts b/src/renderer/src/store/templates.ts
index 79175b0..f94b018 100644
--- a/src/renderer/src/store/templates.ts
+++ b/src/renderer/src/store/templates.ts
@@ -1,10 +1,8 @@
 import { create } from 'zustand'
 import type { CommandTemplate } from '@shared/ipc'
+import { isPreview as PREVIEW } from '../isPreview'
 import { logError } from '../reportError'
 
-// True in the standalone browser preview (no Electron preload).
-const PREVIEW = typeof window === 'undefined' || !window.electron
-
 // Sample templates so the Settings tab has content during UI design.
 const seed: CommandTemplate[] = PREVIEW
   ? [
diff --git a/src/shared/ipc.ts b/src/shared/ipc.ts
index 1fbc978..ce1fd55 100644
--- a/src/shared/ipc.ts
+++ b/src/shared/ipc.ts
@@ -568,6 +568,60 @@ export interface Settings {
   updateToken: string
 }
 
+/**
+ * The canonical default Settings — the single source of truth shared by the main
+ * electron-store defaults (main/settings.ts), the renderer's pre-load FALLBACK
+ * (store/settings.ts), and the browser-preview mock (mockApi.ts), so the full
+ * Settings object is no longer hand-maintained in three places (C1). Preview-only
+ * tweaks (placeholder folders, skip onboarding, demo channel) are layered on top
+ * of this at each call site.
+ */
+export const DEFAULT_SETTINGS: Settings = {
+  // Both blank by default → downloads land in Documents\Video / Documents\Audio
+  // (see getDefaultMediaDir). A non-empty value is an explicit per-kind override.
+  videoDir: '',
+  audioDir: '',
+  defaultKind: 'video',
+  defaultVideoQuality: 'Best available',
+  defaultAudioQuality: 'Best',
+  maxConcurrent: 2,
+  filenameTemplate: '%(title)s.%(ext)s',
+  // Follow the OS theme on first launch (SR1) so a dark-mode Windows user isn't
+  // greeted by a bright white window despite full system-theme support.
+  theme: 'system',
+  accentColor: 'teal',
+  clipboardWatch: true,
+  downloadOptions: DEFAULT_DOWNLOAD_OPTIONS,
+  proxy: '',
+  rateLimit: '',
+  useAria2c: false,
+  cookieSource: 'none',
+  cookiesBrowser: 'chrome',
+  youtubePlayerClient: '',
+  youtubePoToken: '',
+  restrictFilenames: false,
+  downloadArchive: false,
+  // yt-dlp is self-managed: a writable copy under userData, auto-updated on the
+  // configured channel so a stale binary can't silently cause YouTube 403s.
+  autoUpdateYtdlp: true,
+  // Default to stable so a nightly regression doesn't break downloads for
+  // everyone out of the box (SR2). Users who want the latest YouTube fixes
+  // can switch to nightly in Settings → Software.
+  ytdlpChannel: 'stable',
+  ytdlpLastUpdateCheck: 0,
+  customCommandEnabled: false,
+  defaultTemplateId: null,
+  notifyOnComplete: true,
+  // Default off so adding a watched channel doesn't silently fill the user's
+  // disk on first use (SR3). Enable explicitly once they know what it does.
+  autoDownloadNew: false,
+  hasCompletedOnboarding: false,
+  minimizeToTray: false,
+  launchAtStartup: false,
+  sidebarCollapsed: false,
+  updateToken: ''
+}
+
 /**
  * A named, reusable set of extra yt-dlp CLI flags (Phase C "custom commands"),
  * e.g. name: 'Write thumbnail + no mtime', args: '--write-thumbnail --no-mtime'.
@@ -633,6 +687,13 @@ export interface HistoryEntry {
   completedAt: number
 }
 
+/**
+ * Cap on persisted history rows. Single-sourced here so the main store (history.ts)
+ * and the renderer's optimistic in-memory list (store/history.ts, L141) share one
+ * value instead of two hand-synced copies.
+ */
+export const HISTORY_MAX_ENTRIES = 500
+
 /**
  * A failed download or download-start attempt, persisted to errorlog.json
  * (newest-first) so the report survives the queue item being cleared —
diff --git a/test/jsonStore.test.ts b/test/jsonStore.test.ts
index 2ac6ad1..7b4b1e0 100644
--- a/test/jsonStore.test.ts
+++ b/test/jsonStore.test.ts
@@ -1,4 +1,4 @@
-import { describe, it, expect, afterEach } from 'vitest'
+import { describe, it, expect, afterEach, vi } from 'vitest'
 import { mkdtempSync, rmSync, writeFileSync, readdirSync } from 'fs'
 import { tmpdir } from 'os'
 import { join } from 'path'
@@ -49,6 +49,24 @@ describe('jsonStore atomic write + safe read', () => {
     expect(readJsonArraySafe(file, isRow)).toEqual([{ id: 'a' }, { id: 'c' }])
   })
 
+  it('reports success/failure instead of swallowing a bad write (R6)', () => {
+    const d = tmp()
+    const err = vi.spyOn(console, 'error').mockImplementation(() => {})
+    try {
+      // A normal write lands and reports true, with nothing logged.
+      expect(writeJsonAtomic(join(d, 'data.json'), [{ id: 'a' }])).toBe(true)
+      expect(err).not.toHaveBeenCalled()
+      // A target under a non-existent directory can't be written — it must report
+      // false (so a caller can warn/retry) and log, not throw or silently drop it.
+      expect(writeJsonAtomic(join(d, 'no', 'such', 'dir', 'data.json'), [{ id: 'a' }])).toBe(false)
+      expect(err).toHaveBeenCalledOnce()
+    } finally {
+      err.mockRestore()
+    }
+    // No stray .tmp left behind by the failed write.
+    expect(readdirSync(d)).toEqual(['data.json'])
+  })
+
   it('backs up a corrupt file instead of silently wiping it (R2)', () => {
     const d = tmp()
     const file = join(d, 'data.json')