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/main/runtime/cli-command-context-factory.test.ts

@@ -78,6 +78,7 @@ test('cli command context factory composes main deps and context handlers', () =
     openYomitanSettings: () => {},
     openConfigSettingsWindow: () => {},
     openSyncUiWindow: () => {},
+    openYoutubeBrowserWindow: () => {},
     cycleSecondarySubMode: () => {},
     openRuntimeOptionsPalette: () => {},
     printHelp: () => {},