feat(youtube): add YouTube browser window and Whisper subtitle source

- Add `subminer youtube`/`yt` and tray "Browse YouTube" window that plays or queues picked videos in mpv
- Add `youtube.subtitleSource` (`youtube` | `whisper`) to transcribe YouTube audio via subtitle generation
- Allow the subtitle generation modal on YouTube videos
- Pick the real Japanese track over machine-translated ones and fall back to yt-dlp on HTTP 429
- Remove legacy launcher youtubeSubgen/whisper/AI args and `--youtube-mode`
- Fix EPIPE crash when the launching terminal is closed with Ctrl+C
- Thin media timing review handles with orange original-boundary bars
- Update docs and change fragments

src/config/definitions/options-core.ts

@@ -133,6 +133,18 @@ export function buildCoreConfigOptionRegistry(
       description:
         'Comma-separated primary subtitle language priority for managed subtitle auto-selection.',
     },
+    {
+      path: 'youtube.subtitleSource',
+      kind: 'enum',
+      enumValues: ['youtube', 'whisper'],
+      enumLabels: {
+        youtube: 'Download YouTube captions',
+        whisper: 'Generate with Whisper',
+      },
+      defaultValue: defaultConfig.youtube.subtitleSource,
+      description:
+        'Where primary YouTube subtitles come from. Whisper transcribes the audio locally using the subtitleGeneration settings.',
+    },
     {
       path: 'youtube.mediaCache.mode',
       kind: 'enum',