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

launcher/commands/playback-command.ts

@@ -246,7 +246,6 @@ export async function runPlaybackCommandWithDeps(
 
   const isYoutubeUrl = selectedTarget.kind === 'url' && isYoutubeTarget(selectedTarget.target);
   const isAppOwnedYoutubeFlow = isYoutubeUrl;
-  const youtubeMode = args.youtubeMode ?? 'download';
   const configDir = getLauncherConfigDir();
 
   if (isYoutubeUrl) {
@@ -331,7 +330,7 @@ export async function runPlaybackCommandWithDeps(
       );
     }
     const extraAppArgs = isAppOwnedYoutubeFlow
-      ? ['--youtube-play', selectedTarget.target, '--youtube-mode', youtubeMode]
+      ? ['--youtube-play', selectedTarget.target]
       : shouldLauncherAttachRunningApp
         ? [
             pluginRuntimeConfig.autoStartVisibleOverlay