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/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