fix: remove space and / as artist separators, keep &

Separators: • · ; , & — space and / removed to avoid breaking multi-word artist names.

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

backend/novasonic-server.js

@@ -246,8 +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.
-  const splitter = /[•·;,&/]/.test(str) ? /\s*[•·;,&/]\s*/ : /\s+/;
-  return str.split(splitter).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