feat: YouTube/YT Music importer + album artist in view mode

- Info modal view now shows Album-Künstler row from Navidrome albumArtist
- Song Importer detects youtube.com / youtu.be URLs (red badge)
- New /api/youtube/resolve endpoint: yt-dlp --dump-json for single
  videos and playlists (up to 50 entries), enriched with iTunes lookup
  for proper album/year/genre/cover on single tracks
- New src/youtube.js renders preview card and delegates download to
  executeSpotifyDownload (which already handles sourceUrl → direct yt-dlp)
- Importer hint updated to list YouTube as a supported source

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

backend/novasonic-server.js

@@ -240,6 +240,80 @@ app.post('/api/spotify/resolve', async (req, res) => {
   }
 });
 
+// Strip " - Topic" suffix YouTube Music appends to artist channel names
+function _cleanYtArtist(name) {
+  return (name || '').replace(/\s*-\s*Topic$/i, '').trim();
+}
+
+app.post('/api/youtube/resolve', async (req, res) => {
+  const { url } = req.body;
+  if (!url) return res.status(400).json({ error: 'url required' });
+
+  try {
+    const { execFile } = require('child_process');
+    const stdout = await new Promise((resolve, reject) => {
+      execFile(CONFIG.ytdlpBin, [
+        '--dump-json', '--flat-playlist', '--no-warnings',
+        '--playlist-items', '1-50',
+        url,
+      ], { timeout: 45000, maxBuffer: 20 * 1024 * 1024 }, (err, out, stderr) => {
+        if (err && !out?.trim()) return reject(new Error(stderr?.trim() || err.message));
+        resolve(out || '');
+      });
+    });
+
+    const entries = stdout.trim().split('\n')
+      .map(l => { try { return JSON.parse(l.trim()); } catch { return null; } })
+      .filter(Boolean);
+
+    if (!entries.length) return res.status(404).json({ error: 'Keine Ergebnisse von yt-dlp' });
+
+    const isPlaylist = entries.length > 1 || entries[0]?._type === 'playlist';
+
+    if (isPlaylist) {
+      const plTitle = entries[0]?.playlist_title || entries[0]?.playlist || 'YouTube Playlist';
+      const plArtist = _cleanYtArtist(entries[0]?.uploader || entries[0]?.channel || '');
+      const tracks = entries.map(e => ({
+        trackName:  e.title || '',
+        artistName: _cleanYtArtist(e.uploader || e.channel || plArtist),
+        artworkUrl: e.thumbnail || '',
+        durationMs: e.duration ? Math.round(e.duration * 1000) : undefined,
+        sourceUrl:  e.webpage_url || e.url || (e.id ? `https://www.youtube.com/watch?v=${e.id}` : ''),
+      }));
+      return res.json({ type: 'playlist', name: plTitle, artist: plArtist,
+        artworkUrl: entries[0]?.thumbnail || '', tracks });
+    }
+
+    // Single video — enrich with iTunes
+    const e = entries[0];
+    const rawArtist = _cleanYtArtist(e.uploader || e.channel || e.creator || '');
+    const title     = e.title || '';
+    const sourceUrl = e.webpage_url || url;
+    let itunes = {};
+    try { itunes = await lookupItunesTrackMeta(rawArtist.split(',')[0].trim(), title); } catch {}
+
+    const artist     = itunes.artist     || rawArtist;
+    const artworkUrl = itunes.artworkUrl || e.thumbnail || '';
+    return res.json({
+      type: 'track', name: title, artist, artworkUrl,
+      tracks: [{
+        trackName:      title,
+        artistName:     artist,
+        artworkUrl:     artworkUrl,
+        collectionName: itunes.album      || '',
+        releaseDate:    itunes.year       ? String(itunes.year) : '',
+        trackNumber:    itunes.trackNum   || undefined,
+        durationMs:     itunes.durationMs || (e.duration ? Math.round(e.duration * 1000) : undefined),
+        genre:          itunes.genre      || '',
+        sourceUrl,
+      }],
+    });
+  } catch (e) {
+    console.error('[youtube/resolve]', e.message);
+    res.status(500).json({ error: e.message });
+  }
+});
+
 app.post('/api/download', async (req, res) => {
   const { type, artist, title, album, artworkUrl, collectionId, jobId: clientJobId, explicit,
           trackNum: bodyTrackNum, totalTracks: bodyTotalTracks, discNum: bodyDiscNum, discTotal: bodyDiscTotal,