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.