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>

src/api.js

@@ -52,8 +52,15 @@ export async function ndAuth() {
 // ─── ROMAJI INDEX ───
 // Fetched once per session from the backend, cached in memory.
 let _romajiIndex = null;
+let _romajiMap   = null; // Map<songId, { titleRomaji, artistRomaji, albumRomaji }>
 let _romajiIndexFetch = null; // in-flight promise dedup
 
+function _buildRomajiMap(idx) {
+  const m = new Map();
+  if (Array.isArray(idx)) idx.forEach(e => m.set(String(e.id), e));
+  return m;
+}
+
 export async function fetchRomajiIndex() {
   if (!S.cfg.backendUrl || !S.cfg.url || !S.cfg.user || !S.cfg.pass) return null;
   if (_romajiIndex) return _romajiIndex;
@@ -61,12 +68,39 @@ export async function fetchRomajiIndex() {
   _romajiIndexFetch = fetch(
     `${S.cfg.backendUrl}/api/romaji-index?ndUrl=${encodeURIComponent(S.cfg.url)}&user=${encodeURIComponent(S.cfg.user)}&pass=${encodeURIComponent(S.cfg.pass)}`
   ).then(r => r.ok ? r.json() : null)
-   .then(idx => { _romajiIndex = idx; _romajiIndexFetch = null; return idx; })
+   .then(idx => { _romajiIndex = idx; _romajiMap = _buildRomajiMap(idx); _romajiIndexFetch = null; return idx; })
    .catch(() => { _romajiIndexFetch = null; return null; });
   return _romajiIndexFetch;
 }
 
-export function invalidateRomajiIndex() { _romajiIndex = null; }
+export function invalidateRomajiIndex() { _romajiIndex = null; _romajiMap = null; }
+
+// Synchronous lookup — returns index entry for a song ID, or null if not in index.
+export function romajiLookup(id) { return _romajiMap?.get(String(id)) ?? null; }
+
+// Display helpers — return romaji when showRomaji is on and the song is in the index,
+// otherwise fall back to the original Navidrome values.
+export function dispTitle(s) {
+  if (!S.cfg.showRomaji) return s.title || '';
+  return romajiLookup(s.id)?.titleRomaji || s.title || '';
+}
+export function dispArtist(s) {
+  if (!S.cfg.showRomaji) return s.artist || '';
+  return romajiLookup(s.id)?.artistRomaji || s.artist || '';
+}
+export function dispAlbum(s) {
+  if (!S.cfg.showRomaji) return s.album || '';
+  return romajiLookup(s.id)?.albumRomaji || s.album || '';
+}
+// Returns [{name, id}] like parseArtists but with romaji display names when showRomaji is on.
+// Multi-artist romaji is collapsed into one entry since we can't reliably split the converted string.
+export function dispArtistParts(s, parseArtistsFn) {
+  const parts = parseArtistsFn(s);
+  if (!S.cfg.showRomaji) return parts;
+  const info = romajiLookup(s.id);
+  if (!info?.artistRomaji) return parts;
+  return [{ name: info.artistRomaji, id: parts[0]?.id || '' }];
+}
 
 export async function romajiConvert(texts) {
   if (!S.cfg.backendUrl || !texts.length) return texts;