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/main.js

@@ -118,8 +118,7 @@ window.addEventListener('load', () => {
   S.audio.addEventListener('pause', () => {
     setPlayIcon(false);
     document.getElementById('player-art').classList.remove('spinning');
-    // Skip during track transitions — src change fires pause but we're about to play() immediately
-    if(!S._changingTrack && navigator.mediaSession) navigator.mediaSession.playbackState = 'paused';
+    if(navigator.mediaSession) navigator.mediaSession.playbackState = 'paused';
   });
   S.previewAudio.addEventListener('timeupdate', updatePreviewProgress);
   S.previewAudio.addEventListener('ended', stopPreview);