feat: accept & and / as artist separators in metadata editor

_fmtArtists and _normalizeArtists now split on & and / in addition to
• · ; , — so "Arriez & King Dif" typed in the field normalizes correctly.

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

backend/novasonic-server.js

@@ -244,7 +244,7 @@ app.post('/api/spotify/resolve', async (req, res) => {
 // and rejoins with the canonical sep ("; " for track artist, ", " for album artist).
 function _normalizeArtists(str, sep) {
   if (!str) return str;
-  return str.split(/\s*[•·;,]\s*/).map(s => s.trim()).filter(Boolean).join(sep);
+  return str.split(/\s*[•·;,&/]\s*/).map(s => s.trim()).filter(Boolean).join(sep);
 }
 
 // Strip " - Topic" suffix YouTube Music appends to artist channel names