Fix C1/C2 + reliability cluster (L140/L141/L148, R6/R7)

Critical:
- C1: single source of truth for IPC mock + Settings defaults. DEFAULT_SETTINGS
  in @shared/ipc; main DEFAULTS, renderer FALLBACK, and preview MOCK_SETTINGS all
  derive from it. Whole browser mock collapsed into one typed mockApi.ts; main.tsx
  shrinks to window.api = mockApi. PREVIEW check single-sourced in isPreview.ts.
- C2: break the downloads<->sources circular import via a typed event bus
  (store/coordinator.ts). App eagerly imports sources for side-effects so startup
  load + scheduled --sync + downloadCompleted subscription still run.

Reliability:
- L140/L148: cancel/pause release the active slot synchronously; identity-guarded
  releaseActive; per-spawn cookie jar so a same-id retry/resume is never rejected
  by a stale entry nor run over the concurrency cap.
- L141: renderer history capped at 500 + url de-dup to match main.
- R6: writeJsonAtomic reports/logs write failures and keeps data dirty for retry
  instead of an empty catch.
- R7: encryptSecret warns before the plaintext fallback.

typecheck + 243 tests + eslint green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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()