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>

backend/lib/ytdlp.js

@@ -106,8 +106,17 @@ async function downloadTrack({ jobId, artist, albumArtist, title, album, year, t
   // Fetch YTM search result URLs as a flat list (no channel expansion) and return only
   // actual video URLs (watch?v=…). This prevents yt-dlp from following an artist channel
   // result and treating its uploads as the search result set.
+  //
+  // yt-dlp expands "url_transparent" artist channel entries even under --flat-playlist,
+  // printing the channel's video URLs instead of the channel URL. We detect this by
+  // watching for [youtube:tab] in output (which only fires during channel/tab extraction)
+  // and return [] so the caller falls through to the ytsearch fallback.
   const getYtmVideoUrls = (maxItems) => new Promise(resolve => {
     const urls = [];
+    let channelExpanded = false;
+    const checkExpansion = (text) => {
+      if (text.includes('[youtube:tab]') || text.includes('/browse/UC')) channelExpanded = true;
+    };
     const proc = spawn(CONFIG.ytdlpBin, [
       ytmUrl,
       '--flat-playlist',
@@ -116,15 +125,18 @@ async function downloadTrack({ jobId, artist, albumArtist, title, album, year, t
       '--no-warnings',
     ], { env: { ...process.env, PYTHONUNBUFFERED: '1' } });
     proc.stdout.on('data', d => {
-      d.toString().split('\n').forEach(line => {
-        const u = line.trim();
-        // Only accept video URLs — skip /browse/ (artist channel) and /playlist? entries
-        if (u.includes('watch?v=')) urls.push(u);
-      });
+      const text = d.toString();
+      checkExpansion(text);
+      if (!channelExpanded) {
+        text.split('\n').forEach(line => {
+          const u = line.trim();
+          if (u.includes('watch?v=')) urls.push(u);
+        });
+      }
     });
-    proc.stderr.on('data', () => {});
+    proc.stderr.on('data', d => checkExpansion(d.toString()));
     proc.on('error', () => resolve([]));
-    proc.on('close', () => resolve(urls));
+    proc.on('close', () => resolve(channelExpanded ? [] : urls));
   });
 
   // Search YTM for video URLs (pre-filtered), then pass them all at once to yt-dlp.