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/settings.ts

@@ -11,6 +11,7 @@ import {
   COOKIE_BROWSERS,
   ACCENT_COLORS,
   DEFAULT_DOWNLOAD_OPTIONS,
+  isYtdlpUpdateChannel,
   type Settings,
   type DownloadOptions,
   type SponsorBlockCategory
@@ -37,6 +38,11 @@ const DEFAULTS: Settings = {
   cookiesBrowser: 'chrome',
   restrictFilenames: false,
   downloadArchive: false,
+  // yt-dlp is self-managed: a writable copy under userData, auto-updated on the
+  // nightly channel (fastest to follow YouTube changes that cause 403s).
+  autoUpdateYtdlp: true,
+  ytdlpChannel: 'nightly',
+  ytdlpLastUpdateCheck: 0,
   customCommandEnabled: false,
   defaultTemplateId: null,
   notifyOnComplete: true,
@@ -193,12 +199,24 @@ export function setSettings(partial: Partial<Settings>): Settings {
       case 'useAria2c':
       case 'restrictFilenames':
       case 'downloadArchive':
+      case 'autoUpdateYtdlp':
       case 'customCommandEnabled':
       case 'notifyOnComplete':
       case 'autoDownloadNew':
       case 'hasCompletedOnboarding':
         if (typeof value === 'boolean') s.set(key, value)
         break
+      case 'ytdlpChannel':
+        // Same allowlist that guards the `--update-to` flag (audit F1).
+        if (isYtdlpUpdateChannel(value)) s.set('ytdlpChannel', value)
+        break
+      case 'ytdlpLastUpdateCheck': {
+        // Set by the main-process auto-updater; validated here since setSettings
+        // is the only writer. A bogus value at worst skips/forces one check.
+        const n = Number(value)
+        if (Number.isFinite(n) && n >= 0) s.set('ytdlpLastUpdateCheck', n)
+        break
+      }
       case 'defaultTemplateId':
         if (value === null || typeof value === 'string') s.set('defaultTemplateId', value)
         break