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/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
     }