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>

backend/lib/romaji.js

@@ -130,15 +130,18 @@ async function buildRomajiIndex(ndUrl, user, pass) {
   const index = songs
     .filter(s => JP_RE.test(s.title) || JP_RE.test(s.artist) || JP_RE.test(s.album))
     .map(s => ({
-      id:       s.id,
-      r:        [toRomaji(s.title), toRomaji(s.artist), toRomaji(s.album)].join(' '),
-      title:    s.title,
-      artist:   s.artist    || '',
-      album:    s.album     || '',
-      coverArt: s.coverArt  || '',
-      duration: s.duration  || 0,
-      albumId:  s.albumId   || '',
-      artistId: s.artistId  || '',
+      id:           s.id,
+      r:            [toRomaji(s.title), toRomaji(s.artist), toRomaji(s.album)].join(' '),
+      title:        s.title,
+      artist:       s.artist    || '',
+      album:        s.album     || '',
+      titleRomaji:  romajiForStorage(s.title)  || s.title,
+      artistRomaji: romajiForStorage(s.artist) || s.artist || '',
+      albumRomaji:  romajiForStorage(s.album)  || s.album  || '',
+      coverArt:     s.coverArt  || '',
+      duration:     s.duration  || 0,
+      albumId:      s.albumId   || '',
+      artistId:     s.artistId  || '',
     }));
   romajiCache.set(key, { ts: Date.now(), index });
   return { fromCache: false, index };