fix: remove & as artist separator

Separators: • · ; , only.

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

backend/novasonic-server.js

@@ -246,7 +246,7 @@ function _normalizeArtists(str, sep) {
   if (!str) return str;
   // If an explicit separator is present use it; otherwise fall back to splitting on spaces
   // so the user can type "Artist1 Artist2" without a semicolon.
-  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