fix: detect YTM artist channel expansion in getYtmVideoUrls; revert PWA MediaSession changes

- ytdlp.js: watch for [youtube:tab] in yt-dlp output during flat-playlist step;
  return [] when channel expansion is detected so cascade falls through to ytsearch
- Revert player.js / main.js / state.js PWA _changingTrack changes (not effective)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/state.js

@@ -12,7 +12,6 @@ export const S = {
   previewAudio: null,
   previewCurrent: null,
   _scrubbing: false,
-  _changingTrack: false,
   _msPosLast: 0,
   queueOpen: false,