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>

styles.css

@@ -485,6 +485,8 @@ button:focus{outline:none}
 .info-edit-art-btn:hover{color:var(--text)}
 .info-edit-fields{display:flex;flex-direction:column;gap:8px;padding:0 20px 4px}
 .info-edit-label{display:flex;flex-direction:column;font-size:11px;color:var(--text3);gap:3px;font-weight:500}
+.info-edit-hint{font-size:10px;color:var(--text3);font-weight:400;opacity:.7;margin-top:-1px}
+.info-edit-hint code{background:var(--surface3,var(--surface2));border-radius:3px;padding:0 3px;font-size:10px;font-family:monospace;opacity:.9}
 .info-edit-input{background:var(--surface2);border:1px solid var(--border2);border-radius:7px;color:var(--text);font-family:var(--font);font-size:13px;padding:7px 10px;outline:none;width:100%;box-sizing:border-box}
 .info-edit-input:focus{border-color:var(--accent)}