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/defaults-core.ts

@@ -119,6 +119,7 @@ export const CORE_DEFAULT_CONFIG: Pick<
   },
   youtube: {
     primarySubLanguages: ['ja', 'jpn'],
+    subtitleSource: 'youtube',
     mediaCache: {
       mode: 'direct',
       maxHeight: 720,