feat: accept space, &, / as artist separators in metadata editor

- Space as fallback separator on save when no explicit separator present
  (Artist1 Artist2 → Artist1;Artist2)
- & and / added to explicit separator set everywhere
- navidromeArtist (download path) also handles /

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

src/ui.js

@@ -240,7 +240,7 @@ document.addEventListener('click', e => {
 // Track artist → "Serph; Zun"  |  Album artist → "Serph, Zun"
 function _fmtArtists(str, sep) {
   if (!str) return '';
-  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);
 }
 
 function _renderInfoModal(s) {