fix: use first artist only for folder path on multi-artist tracks

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

backend/novasonic-server.js

@@ -163,7 +163,7 @@ async function lookupItunesTrackMeta(artist, title) {
 // Download a single track: yt-dlp audio + ffmpeg clean metadata + iTunes cover embedded
 async function downloadTrack({ jobId, artist, albumArtist, title, album, year, trackNum, totalTracks, discNum, discTotal, compilation, genre, artworkUrl, explicit, outputDir }) {
   const mainArtist = albumArtist || artist;
-  const safeArtist = sanitize(mainArtist);
+  const safeArtist = sanitize(mainArtist.split(',')[0].trim());
   const safeAlbum  = sanitize(album || 'Singles');
   const dir = path.join(outputDir || CONFIG.musicPath, safeArtist, safeAlbum);
   fs.mkdirSync(dir, { recursive: true });