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/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 */
+    }
+  }
 }