feat: artist separator normalization + separator hints in metadata editor

- Normalize •, ·, ;, , separators on save (both edit and download paths)
- Fix ndSong scope bug (was inaccessible outside try block in edit-metadata)
- Apply _normalizeArtists to artist/albumArtist before ffmpeg tags and file rename
- Storage format: Artist A;Artist B (no space, Navidrome/MusicBrainz style)
- Display format: Artist A; Artist B (space, via _fmtArtists in frontend)
- Show separator hint below label in metadata editor fields
- Remove hover effect on player bar song title

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

backend/lib/metadata.js

@@ -13,7 +13,7 @@ function navidromeArtist(name) {
     .replace(/\s+ft\.?\s+/gi, ';')
     .replace(/\s+featuring\s+/gi, ';')
     .replace(/\s+&\s+/g, ';')
-    .replace(/,\s+/g, ';');
+    .replace(/,\s*/g, ';');
 }
 
 // Download hi-res iTunes artwork (1000x1000) to cover.jpg, return path or null