fix(spotify): use ' / ' as artist separator for Navidrome multi-artist splitting

Navidrome splits artist tags on '/' (Tags.artist.Split = ["/"]).
Using ' / ' between multiple artists (both from array format and from
Spotify's comma-separated subtitle) lets Navidrome register them as
separate artists without any config change.

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

backend/novasonic-server.js

@@ -439,7 +439,8 @@ function _spAllArtists(artistsRaw) {
   if (!artistsRaw) return '';
   // Handle both array format and GraphQL {items:[...]} format
   const arr = Array.isArray(artistsRaw) ? artistsRaw : (artistsRaw.items || []);
-  return arr.map(a => a?.profile?.name || a?.name || '').filter(Boolean).join(', ');
+  // Use " / " so Navidrome splits on "/" into multiple artists
+  return arr.map(a => a?.profile?.name || a?.name || '').filter(Boolean).join(' / ');
 }
 
 function _spArtistName(artistsRaw) {
@@ -461,7 +462,8 @@ function _spTrackItem(t, defaultAlbum, defaultArt, defaultAlbumArtist) {
   if (d?.artists) {
     artist = _spAllArtists(d.artists);
   } else if (typeof d?.subtitle === 'string' && d.subtitle) {
-    artist = d.subtitle;
+    // Spotify subtitle separates artists with ", " — convert to " / " for Navidrome splitting
+    artist = d.subtitle.replace(/,\s*/g, ' / ');
   }
   artist = artist || defaultAlbumArtist || '';