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/preload/index.ts

@@ -7,6 +7,8 @@ import {
   type YtdlpVersionResult,
   type YtdlpUpdateChannel,
   type YtdlpUpdateResult,
+  type YtdlpAutoUpdateStatus,
+  type FfmpegVersionResult,
   type ProbeResult,
   type StartDownloadOptions,
   type StartDownloadResult,
@@ -55,6 +57,10 @@ const api = {
   getYtdlpVersion: (): Promise<YtdlpVersionResult> =>
     ipcRenderer.invoke(IpcChannels.ytdlpVersion),
 
+  /** Versions of the bundled ffmpeg + ffprobe (display-only; not auto-updated). */
+  getFfmpegVersions: (): Promise<FfmpegVersionResult> =>
+    ipcRenderer.invoke(IpcChannels.ffmpegVersion),
+
   probe: (url: string): Promise<ProbeResult> => ipcRenderer.invoke(IpcChannels.probe, url),
 
   startDownload: (opts: StartDownloadOptions): Promise<StartDownloadResult> =>
@@ -115,6 +121,13 @@ const api = {
   updateYtdlp: (channel: YtdlpUpdateChannel): Promise<YtdlpUpdateResult> =>
     ipcRenderer.invoke(IpcChannels.ytdlpUpdate, channel),
 
+  /** Subscribe to background yt-dlp auto-update status. Returns an unsubscribe function. */
+  onYtdlpAutoUpdateStatus: (cb: (s: YtdlpAutoUpdateStatus) => void): (() => void) => {
+    const listener = (_e: IpcRendererEvent, s: YtdlpAutoUpdateStatus): void => cb(s)
+    ipcRenderer.on(IpcChannels.ytdlpAutoUpdateStatus, listener)
+    return () => ipcRenderer.removeListener(IpcChannels.ytdlpAutoUpdateStatus, listener)
+  },
+
   listErrorLog: (): Promise<ErrorLogEntry[]> => ipcRenderer.invoke(IpcChannels.errorLogList),
 
   clearErrorLog: (): Promise<ErrorLogEntry[]> => ipcRenderer.invoke(IpcChannels.errorLogClear),