feat: self-managing yt-dlp + ffmpeg version display

yt-dlp is now a managed, auto-updating binary rather than a static bundled
one. On launch AeroFetch seeds a writable copy under userData from the
bundled seed, self-heals it if it goes missing, and — throttled to once a
day — self-updates it to the configured channel (default: nightly). This
keeps the binary current so YouTube changes don't silently cause 403s, and
an app reinstall (or portable re-extraction) can no longer roll it back.
Settings shows an auto-update toggle, the live version, last-checked time,
and the channel selector.

Also surface the bundled ffmpeg/ffprobe versions in Settings (display only:
they have no self-update path and, unlike yt-dlp, don't break as sites
change, so there is no auto-update for them).

- binaries: split the read-only bundled seed from the managed userData copy
- ytdlp: ensureManagedYtdlp (seed + self-heal), startup auto-update runner
- ytdlpPolicy: pure once-a-day throttle decision (unit-tested)
- ffmpeg: parse ffmpeg/ffprobe -version for the Settings panel
- settings/ipc/preload/renderer: new settings, IPC channels, types, and UI

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/main/ytdlpPolicy.ts

@@ -0,0 +1,25 @@
+/**
+ * Pure auto-update scheduling policy for yt-dlp. Kept free of any electron /
+ * node-runtime dependency (like buildArgs.ts) so the throttle can be unit-tested
+ * without spinning up Electron. ytdlp.ts imports these to drive the real check.
+ */
+
+/** Once-a-day throttle for the startup auto-update check. */
+export const AUTO_UPDATE_INTERVAL_MS = 24 * 60 * 60 * 1000
+
+/**
+ * Whether a background yt-dlp update check should run now: only when auto-update
+ * is enabled AND at least one interval has elapsed since the last check. A
+ * zero/absent lastCheck means "never checked" → run. `now` and the interval are
+ * passed in so the decision is a pure function of its inputs.
+ */
+export function shouldAutoCheckYtdlp(
+  enabled: boolean,
+  lastCheck: number,
+  now: number,
+  intervalMs: number = AUTO_UPDATE_INTERVAL_MS
+): boolean {
+  if (!enabled) return false
+  if (!lastCheck) return true
+  return now - lastCheck >= intervalMs
+}