feat: keep Japanese in main tags, romaji in sort tags + frontend toggle

- Downloads now store original Japanese in TITLE/ARTIST/ALBUM tags
  so Apple Music, iTunes, YTM etc. all see the authentic original text
- Sort tags (TITLESORT/ARTISTSORT/ALBUMSORT) written with romaji for
  apps that use them for sorting/display
- iTunes-resolved English artist name used for ARTISTSORT and folder names
  when available (more accurate than kuromoji for known artists)
- rawArtist parameter added to downloadTrack so YTM search query uses
  the original Japanese channel name (fixes wrong song downloads)
- spotify.js: artistName preserved as original; itunesArtistName stored
  separately so the Spotify importer shows Japanese titles
- romaji index now includes titleRomaji/artistRomaji/albumRomaji fields
- Frontend: showRomaji setting (Settings > Bibliothek) — when on, player
  bar, now-playing, queue, and song lists all show romaji via index lookup
- refreshDisplayMode() updates all live UI when the toggle is saved

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

index.html

@@ -656,6 +656,11 @@
           <div id="s-show-playcount" class="toggle" onclick="this.classList.toggle('on')"></div>
         </div>
         <div class="hint" style="margin-top:6px">Zeigt Wiedergabezahl in Album- und Playlist-Ansichten. Künstlerseite zeigt sie immer.</div>
+        <div class="edit-pl-toggle-row" style="margin-top:12px">
+          <span>Romaji anzeigen</span>
+          <div id="s-show-romaji" class="toggle" onclick="this.classList.toggle('on')"></div>
+        </div>
+        <div class="hint" style="margin-top:6px">Zeigt japanische Titel und Künstlernamen in Romaji (lateinische Umschrift). Originaltitel bleiben in den Datei-Tags erhalten.</div>
       </div>
       <div class="settings-section">
         <h3 style="display:flex;align-items:center;gap:8px;margin-bottom:12px">