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/cli/help.ts

@@ -80,6 +80,9 @@ ${B}Jellyfin${R}
   --jellyfin-audio-stream-index ${D}N${R}       Audio stream override
   --jellyfin-subtitle-stream-index ${D}N${R}    Subtitle stream override
 
+${B}YouTube${R}
+  --youtube-browser              Open the YouTube browser ${D}(videos play in mpv)${R}
+
 ${B}Stats sync${R}
   --sync-window                  Open the stats sync window
   --sync-cli sync ${D}[host] [opts]${R}   Headless stats sync ${D}(same commands as "subminer sync";${R}