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/player.js

@@ -17,14 +17,8 @@ export function playQueue(songs, startIdx) {
 export function playCurrent() {
   if(S.qIdx < 0 || S.qIdx >= S.queue.length) return;
   const s = S.queue[S.qIdx];
-  // Pre-update MediaSession before src change so the OS never sees a "paused" flash
-  // (setting src fires pause synchronously; _changingTrack suppresses that in the pause handler)
-  updateMediaSession(s);
-  if(navigator.mediaSession) navigator.mediaSession.playbackState = 'playing';
-  S._changingTrack = true;
   S.audio.src = streamUrl(s.id);
-  S._changingTrack = false;
-  S.audio.play().catch(() => {});
+  S.audio.play();
   updatePlayerUI(s);
   onNpSongChange(s.id);
   renderQueue();