fix(spotify): artist split + playlist cover in downloads

navidromeArtist: replace ', ' with ' / ' so Navidrome splits
multi-artist strings (e.g. 'Artist A, Artist B' → 'Artist A / Artist B')
with existing Tags.artist.Split = ["/"] config.

Playlist tracks: remove playlist cover URL as defaultArt in _spTrackItem
so that songs where iTunes lookup fails get no embedded artwork instead
of the wrong playlist cover art.

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

backend/novasonic-server.js

@@ -56,14 +56,15 @@ function sanitize(s) {
   return (s || 'Unknown').replace(/[\\/:*?"<>|]/g, '_').trim().slice(0, 100);
 }
 
-// Convert "Artist feat. B" / "Artist ft. B" / "Artist featuring B" → "Artist / B"
-// so Navidrome splits on "/" and lists each as a separate artist
+// Convert multi-artist strings to " / " separator so Navidrome splits correctly.
+// Handles: "feat." / "ft." / "featuring" and comma-separated artists from Spotify/iTunes.
 function navidromeArtist(name) {
   if (!name) return name;
   return name
     .replace(/\s+feat\.?\s+/gi, ' / ')
     .replace(/\s+ft\.?\s+/gi, ' / ')
-    .replace(/\s+featuring\s+/gi, ' / ');
+    .replace(/\s+featuring\s+/gi, ' / ')
+    .replace(/,\s+/g, ' / ');  // "Artist A, Artist B" → "Artist A / Artist B"
 }
 
 function updateJob(id, update) {
@@ -548,8 +549,9 @@ app.post('/api/spotify/resolve', async (req, res) => {
         artist: owner,
         artworkUrl: artUrl,
         description: entity.description || '',
-        // pass artUrl as defaultArt so tracks show the playlist cover when no per-track art exists
-        tracks: trackList.map(t => _spTrackItem(t, null, artUrl, null)),
+        // no defaultArt for playlist tracks — iTunes lazy-loader will fill correct per-track art;
+        // passing playlist cover here would cause it to be embedded in downloads for unresolved tracks
+        tracks: trackList.map(t => _spTrackItem(t, null, null, null)),
       });
     }