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/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 {