feat(main): CC8 — file-backed leveled logger + renderer log sink

Diagnostics were invisible in a packaged build: main catches did a bare
console.error (DevTools suppressed in prod, M31) and the renderer's logError
(M29) wrote to an unreachable console. Add a small leveled logger
(src/main/logger.ts) appending to <userData>/logs/aerofetch.log with size-capped
rotation; all app/fs access is lazy+guarded so it's a safe no-op under tests.

- Route the 5 main-process console.* catch sites (jsonStore write-fail, settings
  write-fail + plaintext-secret warn, cookie loadURL, yt-dlp auto-update)
  through the logger.
- Add a fire-and-forget log:write IPC channel + preload logError() so the
  renderer's logError forwards failures to the main file sink (closes the M29
  "sink" half the code comments deferred to CC8). mockApi + Api type updated.
- Unit-test pure formatLine/composeMessage + no-op safety; update the R6
  jsonStore test to assert against the logger.

The user-facing toast half of CC8 ties to the global status surface (UI25/UX9).
typecheck + 258 tests + lint + prettier green.

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

src/main/constants.ts

@@ -46,6 +46,13 @@ export const INDEX_MAX_BUFFER = 256 * 1024 * 1024
 /** How much of a failed download's stderr to retain for the error message. */
 export const STDERR_TAIL_BYTES = 4000
 
+/**
+ * Size cap for the diagnostic log file (logger.ts, CC8). Once the live
+ * `aerofetch.log` passes this, it's rotated to `aerofetch.log.1` (one generation
+ * kept) so app logging can't fill the disk.
+ */
+export const LOG_MAX_BYTES = 1024 * 1024
+
 // --- JSON-store row caps ----------------------------------------------------
 // Each hand-rolled JSON store trims to its cap on write so a store file can't
 // grow without bound.

src/main/cookies.ts

@@ -2,6 +2,7 @@ import { app, safeStorage, session, BrowserWindow, type Cookie, type WebContents
 import { existsSync, statSync, unlinkSync, writeFileSync, readFileSync } from 'fs'
 import { join } from 'path'
 import { assertHttpUrl } from './url'
+import { logger } from './logger'
 import type { CookiesStatus, CookiesLoginResult } from '@shared/ipc'
 
 /**
@@ -228,9 +229,7 @@ export function openCookieLoginWindow(
 
     if (loginWindow && !loginWindow.isDestroyed()) {
       pendingResolvers.push(resolve)
-      loginWindow
-        .loadURL(validUrl)
-        .catch((e) => console.error('[AeroFetch] cookie login loadURL failed:', e))
+      loginWindow.loadURL(validUrl).catch((e) => logger.error('cookie login loadURL failed', e))
       loginWindow.focus()
       return
     }

src/main/index.ts

@@ -21,6 +21,7 @@ import { flushAllStores } from './jsonStore'
 import { extractIncomingUrl, registerSendToShortcut, focusWindow } from './deeplink'
 import { isSyncLaunch } from './schedule'
 import { createTray, markQuitting, isQuitting } from './tray'
+import { logger } from './logger'
 
 // Only one instance ever runs. A second launch — e.g. the OS invoking us again
 // for an aerofetch:// link or a "Send to AeroFetch" file — hands its argv to
@@ -271,7 +272,7 @@ if (isPrimaryInstance) {
       if (mainWindow && !mainWindow.isDestroyed()) {
         mainWindow.webContents.send(IpcChannels.ytdlpAutoUpdateStatus, status)
       }
-    }).catch((e) => console.error('[AeroFetch] yt-dlp auto-update failed:', e))
+    }).catch((e) => logger.error('yt-dlp auto-update failed', e))
   })
 
   // Any real quit path (tray "Quit", OS shutdown, the updater's app.quit) must set

src/main/ipc.ts

@@ -50,6 +50,7 @@ import { syncWatchedSources } from './sync'
 import { getScheduledSync, setScheduledSync } from './schedule'
 import { getActiveBadge, getErrorBadge } from './badge'
 import { openPoTokenWindow } from './poToken'
+import { logger } from './logger'
 
 // --- Theme helpers (shared with index's system-theme bridge) ----------------
 
@@ -87,6 +88,12 @@ export function registerIpcHandlers(getMainWindow: () => BrowserWindow | null):
     app.quit()
   })
 
+  // CC8: persist renderer-side failures (the M29 `logError` sites) to the main log
+  // file — the renderer console is unreachable in a packaged build. Fire-and-forget.
+  ipcMain.on(IpcChannels.logWrite, (_e, op: string, detail: string) =>
+    logger.error(`[renderer] ${op}`, detail)
+  )
+
   ipcMain.handle(IpcChannels.appVersion, () => app.getVersion())
 
   ipcMain.handle(IpcChannels.appUpdateCheck, () => checkForAppUpdate())

src/main/jsonStore.ts

@@ -1,6 +1,7 @@
 import { app } from 'electron'
 import { join } from 'path'
 import { readFileSync, writeFileSync, renameSync, existsSync, copyFileSync, unlinkSync } from 'fs'
+import { logger } from './logger'
 
 /**
  * One shared persistence layer for the hand-rolled JSON array stores (history,
@@ -69,9 +70,9 @@ export function writeJsonAtomic(path: string, value: unknown, pretty = true): bo
     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)
+    // profile) dropped the change with no trace. It now lands in the file-backed
+    // diagnostic log (CC8); the user-facing toast ties to the global status surface (UI25/UX9).
+    logger.error(`failed to write ${path}`, e)
     try {
       if (existsSync(tmp)) unlinkSync(tmp)
     } catch {

src/main/logger.ts

@@ -0,0 +1,106 @@
+import { app } from 'electron'
+import { join } from 'path'
+import { appendFileSync, mkdirSync, renameSync, statSync } from 'fs'
+import { LOG_MAX_BYTES } from './constants'
+
+/**
+ * One small leveled, file-backed logger for the whole app (audit CC8). Before this,
+ * diagnostics were effectively invisible in a packaged build: main-process catches
+ * did a bare `console.error` (unreachable — the app menu/DevTools are suppressed in
+ * production, M31) and the renderer's `logError` (M29) wrote to a console nobody could
+ * open. Every catch now routes here and lands in `<userData>/logs/aerofetch.log`, so a
+ * field-reported failure is actually diagnosable.
+ *
+ * Scope: this is the diagnostic log. `errorlog.ts` remains separate — it's user-facing
+ * *download* failure data shown in Settings → Diagnostics, not app logging. The
+ * user-facing toast half of CC8 ties to the global status surface (UI25/UX9).
+ *
+ * All `app`/fs access is lazy and guarded so importing this module never touches
+ * `app` before it's ready — and so it's a safe no-op under unit tests, where
+ * `electron`'s `app` is undefined.
+ */
+
+export type LogLevel = 'error' | 'warn' | 'info' | 'debug'
+
+const LEVELS: Record<LogLevel, number> = { error: 0, warn: 1, info: 2, debug: 3 }
+
+// Write everything at or above this level to the file. `debug` is dev-only noise, so
+// it never hits the file (but still prints to the dev console).
+const FILE_THRESHOLD = LEVELS.info
+
+/** Format one log line: `2026-07-01T08:30:00.000Z [ERROR] message`. Pure, for tests. */
+export function formatLine(level: LogLevel, message: string, now: Date = new Date()): string {
+  return `${now.toISOString()} [${level.toUpperCase()}] ${message}`
+}
+
+/** Combine an operation label with an optional error/detail into one message. Pure. */
+export function composeMessage(label: string, detail?: unknown): string {
+  if (detail === undefined) return label
+  const text = detail instanceof Error ? detail.message : String(detail)
+  return `${label}: ${text}`
+}
+
+// `undefined` = not yet resolved, `null` = resolution failed (no app/userData, e.g.
+// under unit tests) so file logging is disabled; a string is the resolved path.
+let cachedFile: string | null | undefined
+function logFile(): string | null {
+  if (cachedFile !== undefined) return cachedFile
+  try {
+    const dir = join(app.getPath('userData'), 'logs')
+    mkdirSync(dir, { recursive: true })
+    cachedFile = join(dir, 'aerofetch.log')
+  } catch {
+    // No app / userData available (unit test, or app not ready) — disable the file sink.
+    cachedFile = null
+  }
+  return cachedFile
+}
+
+/** Dev mode, resolved lazily and guarded (app is undefined in the test env). */
+function isDev(): boolean {
+  try {
+    return !app.isPackaged
+  } catch {
+    return false
+  }
+}
+
+// Rotate to `<file>.1` (single generation) once the live file passes the cap, so the
+// log can't grow without bound. Best-effort: any fs error here is swallowed — logging
+// must never throw into a catch handler.
+function rotateIfNeeded(file: string): void {
+  try {
+    if (statSync(file).size > LOG_MAX_BYTES) renameSync(file, `${file}.1`)
+  } catch {
+    /* file missing (first write) or rename race — ignore */
+  }
+}
+
+function writeToFile(line: string): void {
+  const file = logFile()
+  if (!file) return
+  rotateIfNeeded(file)
+  try {
+    appendFileSync(file, line + '\n')
+  } catch {
+    /* disk full / read-only profile — nothing more we can safely do */
+  }
+}
+
+function emit(level: LogLevel, label: string, detail?: unknown): void {
+  const message = composeMessage(label, detail)
+  // Console in dev keeps the familiar `--inspect` workflow; the file is the sink that
+  // survives into a packaged build.
+  if (isDev()) {
+    const fn = level === 'error' ? console.error : level === 'warn' ? console.warn : console.log
+    fn(`[AeroFetch] ${message}`)
+  }
+  if (LEVELS[level] <= FILE_THRESHOLD) writeToFile(formatLine(level, message))
+}
+
+export const logger = {
+  error: (label: string, detail?: unknown): void => emit('error', label, detail),
+  warn: (label: string, detail?: unknown): void => emit('warn', label, detail),
+  info: (label: string, detail?: unknown): void => emit('info', label, detail),
+  debug: (label: string, detail?: unknown): void => emit('debug', label, detail)
+}

src/main/settings.ts

@@ -1,6 +1,7 @@
 import { app, safeStorage } from 'electron'
 import Store from 'electron-store'
 import { isSafeFilenameTemplate, isSafeOutputDir } from './validation'
+import { logger } from './logger'
 import {
   AUDIO_FORMATS,
   VIDEO_CONTAINERS,
@@ -130,7 +131,7 @@ function encryptSecret(plain: string): string {
   // 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.')
+  logger.warn('OS secret encryption unavailable — storing credential as plaintext')
   return plain
 }
 
@@ -238,7 +239,7 @@ export function setSettings(partial: Partial<Settings>): Settings {
     // handler or surface as an unhandled rejection. Log it; getSettings() below
     // returns the store's ACTUAL persisted state, so the renderer's reconciliation
     // (M34) reflects what truly saved instead of the optimistic value.
-    console.error('[AeroFetch] settings write failed:', e)
+    logger.error('settings write failed', e)
   }
   cachedSettings = null
   return getSettings()

src/preload/index.ts

@@ -83,6 +83,10 @@ const api = {
 
   readClipboard: (): Promise<string> => ipcRenderer.invoke(IpcChannels.clipboardRead),
 
+  /** Forward a renderer-side failure to the main diagnostic log (CC8). Fire-and-forget. */
+  logError: (op: string, detail: string): void =>
+    ipcRenderer.send(IpcChannels.logWrite, op, detail),
+
   getSettings: (): Promise<Settings> => ipcRenderer.invoke(IpcChannels.settingsGet),
 
   setSettings: (partial: Partial<Settings>): Promise<Settings> =>

src/renderer/src/mockApi.ts

@@ -137,6 +137,7 @@ export const mockApi: Api = {
   openUrl: async () => {},
   showInFolder: async () => {},
   readClipboard: async () => '',
+  logError: () => {},
   getSettings: async () => MOCK_SETTINGS,
   setSettings: async (partial) => Object.assign(MOCK_SETTINGS, partial),
   listHistory: async () => [],

src/renderer/src/reportError.ts

@@ -4,10 +4,18 @@
 // Replaces the swallowed `.catch(() => {})` sites the audit flagged (M29) with a
 // single, consistent log format. `op` names the operation, e.g. 'addHistory'.
 //
-// Note: in production the DevTools console isn't reachable (the app menu is
-// suppressed), so this is primarily a development/`--inspect` diagnostic until a
-// persistent renderer log sink lands (CC8). It is still strictly better than
-// discarding the error.
+// The failure is both printed to the renderer console (dev/`--inspect`) and
+// forwarded to the main process's file-backed diagnostic log (CC8), so it survives
+// into a packaged build where the renderer console is unreachable.
 export function logError(op: string): (e: unknown) => void {
-  return (e) => console.error(`[AeroFetch] ${op} failed:`, e)
+  return (e) => {
+    console.error(`[AeroFetch] ${op} failed:`, e)
+    try {
+      const detail = e instanceof Error ? e.message : String(e)
+      // window.api is absent in the browser UI preview and in unit tests.
+      if (typeof window !== 'undefined') window.api?.logError?.(op, detail)
+    } catch {
+      /* logging must never throw out of a catch handler */
+    }
+  }
 }

src/shared/ipc.ts

@@ -55,6 +55,9 @@ export const IpcChannels = {
   /** preload → main: the contextBridge failed to expose the API; main shows a
    *  hard error instead of letting the renderer silently fall back to mock mode (M30) */
   preloadBridgeFailure: 'preload:bridge-failure',
+  /** renderer → main fire-and-forget: forward a renderer-side failure to the main
+   *  diagnostic log so it survives into a packaged build (CC8; the M29 sink half). */
+  logWrite: 'log:write',
   /** main → renderer push channel: a URL handed to AeroFetch from outside the app
    *  (the aerofetch:// protocol, or a .url file via Explorer's "Send to" menu) */
   externalUrl: 'app:external-url',