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/download.ts

@@ -11,6 +11,7 @@ import {
   getSystem32Path
 } from './binaries'
 import { getSettings, getDownloadArchivePath, getDefaultMediaDir } from './settings'
+import { ensureManagedYtdlp } from './ytdlp'
 import { getCookiesFilePath } from './cookies'
 import { listTemplates } from './templates'
 import { assertHttpUrl } from './url'
@@ -240,11 +241,14 @@ export function startDownload(
   wc: WebContents,
   opts: StartDownloadOptions
 ): StartDownloadResult {
+  // Self-heal the managed copy from the bundled seed before spawning, so a
+  // never-seeded or deleted yt-dlp.exe doesn't fail an otherwise-fine download.
+  ensureManagedYtdlp()
   const ytdlp = getYtdlpPath()
   if (!existsSync(ytdlp)) {
     return {
       ok: false,
-      error: `yt-dlp.exe not found at ${ytdlp}\nDrop it into resources/bin/ (see the README there).`
+      error: `yt-dlp.exe is missing and couldn't be restored from the bundle.\nReinstall AeroFetch, or drop yt-dlp.exe into resources/bin/ (see the README there).`
     }
   }
   // ffmpeg is used by nearly every download (merge, audio extract, thumbnail/