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>

index.html

@@ -928,8 +928,8 @@
       </div>
       <div class="info-edit-fields">
         <label class="info-edit-label">Titel<input class="info-edit-input" id="ie-title" type="text" placeholder="Titel"></label>
-        <label class="info-edit-label">Künstler<input class="info-edit-input" id="ie-artist" type="text" placeholder="Künstler (mit ; trennen)"></label>
-        <label class="info-edit-label">Album-Künstler<input class="info-edit-input" id="ie-albumartist" type="text" placeholder="Album-Künstler (mit , trennen)"></label>
+        <label class="info-edit-label">Künstler<span class="info-edit-hint">Mehrere Künstler mit <code>;</code> trennen — z.B. <code>Künstler A; Künstler B</code></span><input class="info-edit-input" id="ie-artist" type="text" placeholder="Künstler"></label>
+        <label class="info-edit-label">Album-Künstler<span class="info-edit-hint">Mehrere mit <code>,</code> trennen — z.B. <code>Künstler A, Künstler B</code></span><input class="info-edit-input" id="ie-albumartist" type="text" placeholder="Album-Künstler"></label>
         <label class="info-edit-label">Album<input class="info-edit-input" id="ie-album" type="text" placeholder="Album"></label>
         <label class="info-edit-label">Genre<input class="info-edit-input" id="ie-genre" type="text" placeholder="Genre"></label>
         <div style="display:grid;grid-template-columns:1fr 1fr;gap:8px">