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/ui.js

@@ -3,7 +3,7 @@ import { esc, formatDur, toast } from './utils.js';
 import { api, coverUrl, ndAuth, fetchRomajiIndex, romajiConvert } from './api.js';
 import { THEMES, applyTheme, getCurrentThemeId } from './themes.js';
 // Circular imports — safe (only used inside function bodies)
-import { playQueue, dlSongLocal, toggleStar, togglePlay, renderQueue, syncNpStar, updateStarBtn, npToggleStar, playCurrent, _syncNpDirect, setPlayIcon, nativeInsertAt } from './player.js';
+import { playQueue, dlSongLocal, toggleStar, togglePlay, renderQueue, syncNpStar, updateStarBtn, npToggleStar, playCurrent, _syncNpDirect, setPlayIcon, nativeInsertAt, refreshDisplayMode } from './player.js';
 import { navigate, loadPlaylists } from './views.js';
 
 // ─── SIDEBAR ───
@@ -60,6 +60,7 @@ export function openSettings() {
   document.getElementById('s-lastfm').value       = S.cfg.lastfmKey        || '';
   document.getElementById('s-backend').value      = S.cfg.backendUrl       || '';
   document.getElementById('s-show-playcount').classList.toggle('on', !!S.cfg.showPlayCount);
+  document.getElementById('s-show-romaji').classList.toggle('on', !!S.cfg.showRomaji);
   _renderThemePicker();
   document.getElementById('settings-modal').classList.add('open');
   checkBackend();
@@ -92,9 +93,15 @@ export function saveSettings() {
   if(pass) S.cfg.pass = pass;
   S.cfg.lastfmKey        = document.getElementById('s-lastfm').value.trim();
   S.cfg.backendUrl       = document.getElementById('s-backend').value.trim().replace(/\/$/,'');
-  S.cfg.showPlayCount    = document.getElementById('s-show-playcount').classList.contains('on');
+  S.cfg.showPlayCount = document.getElementById('s-show-playcount').classList.contains('on');
+  const romajiChanged = S.cfg.showRomaji !== document.getElementById('s-show-romaji').classList.contains('on');
+  S.cfg.showRomaji    = document.getElementById('s-show-romaji').classList.contains('on');
   localStorage.setItem('ns_cfg', JSON.stringify(S.cfg));
   closeSettings(); toast('✅ Gespeichert');
+  if (romajiChanged) {
+    // Pre-load the index so display is instant, then refresh all live UI
+    fetchRomajiIndex().then(() => refreshDisplayMode());
+  }
 }
 export async function checkBackend() {
   const dot = document.getElementById('backend-status-dot');