fix: use playlist/album artworkUrl as fallback when track has no cover

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

backend/novasonic-server.js

@@ -289,7 +289,7 @@ app.post('/api/download', async (req, res) => {
               trackNum:    itunes.trackNum    || track.trackNumber    || undefined,
               totalTracks: itunes.totalTracks || undefined,
               genre:       itunes.genre       || track.primaryGenreName || undefined,
-              artworkUrl:  itunes.artworkUrl  || track.artworkUrl100  || '',
+              artworkUrl:  itunes.artworkUrl  || track.artworkUrl100  || artworkUrl || '',
               explicit:    track.trackExplicitness === 'explicit',
               durationMs:  itunes.durationMs  || track.durationMs    || undefined,
             });