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/novasonic-server.js

@@ -373,7 +373,7 @@ app.post('/api/download', async (req, res) => {
           log(jobId, `📋 Album aus Spotify: "${meta.album}"`);
         }
         const finalArtist = meta.itunesArtistName || artist;
-        const result = await downloadTrack({ jobId, artist: finalArtist, title, ...meta, sourceUrl: sourceUrl || sp.sourceUrl || undefined });
+        const result = await downloadTrack({ jobId, artist: finalArtist, rawArtist: artist, title, ...meta, sourceUrl: sourceUrl || sp.sourceUrl || undefined });
         if (result?.skipped) {
           updateJob(jobId, { status: 'done', progress: 100, message: 'Bereits vorhanden – übersprungen', skipped: true });
         } else {
@@ -447,10 +447,13 @@ app.post('/api/download', async (req, res) => {
               log(jobId, `📋 iTunes: "${trackArtist} – ${trackTitle}"`);
               itunes = await lookupItunesTrackMeta(searchArtist, trackTitle);
             }
-            const resolvedArtist = itunes.itunesArtistName || trackArtist;
+            // Prefer iTunes-resolved name (from server lookup or frontend enrichment) for sort tags / folders.
+            // trackArtist stays as rawArtist for main tags and YTM search.
+            const resolvedArtist = itunes.itunesArtistName || track.itunesArtistName || trackArtist;
             return downloadTrack({
               jobId,
-              artist:      resolvedArtist,
+              artist:      resolvedArtist, // iTunes English name → used for sort tags + folder names
+              rawArtist:   trackArtist,    // original Spotify/Japanese name → used for main tag + YTM search
               albumArtist: resolvedArtist,
               title:       trackTitle,
               album:       itunes.album       || track.collectionName || track.album || '',
@@ -689,11 +692,18 @@ app.post('/api/edit-metadata', async (req, res) => {
         ffArgs.push('-map', '0:v?'); // '?' = optional, no error if file has no cover
       }
       ffArgs.push('-c', 'copy', '-map_metadata', '-1', '-id3v2_version', '3');
-      if (romajiTitle)       ffArgs.push('-metadata', `title=${romajiTitle}`);
-      if (romajiArtist)      ffArgs.push('-metadata', `artist=${romajiArtist}`);
-      const resolvedAlbumArtist = romajiAlbumArtist || (romajiArtist ? romajiArtist.split(';')[0].trim() : null);
-      if (resolvedAlbumArtist)   ffArgs.push('-metadata', `album_artist=${resolvedAlbumArtist}`);
-      if (romajiAlbum) ffArgs.push('-metadata', `album=${romajiAlbum}`);
+      // Main tags: store original user input (may be Japanese — Apple Music / other apps see authentic text)
+      if (title)         ffArgs.push('-metadata', `title=${title}`);
+      if (normalArtist)  ffArgs.push('-metadata', `artist=${normalArtist}`);
+      const resolvedAlbumArtist = normalAlbumArtist || (normalArtist ? normalArtist.split(';')[0].trim() : null);
+      if (resolvedAlbumArtist) ffArgs.push('-metadata', `album_artist=${resolvedAlbumArtist}`);
+      if (album)         ffArgs.push('-metadata', `album=${album}`);
+      // Sort tags: romaji versions for apps that use them for sorting / display
+      if (romajiTitle       && romajiTitle       !== title)               ffArgs.push('-metadata', `TITLESORT=${romajiTitle}`);
+      if (romajiArtist      && romajiArtist      !== normalArtist)        ffArgs.push('-metadata', `ARTISTSORT=${romajiArtist}`);
+      const resolvedRomajiAA = romajiAlbumArtist || (romajiArtist ? romajiArtist.split(';')[0].trim() : null);
+      if (resolvedRomajiAA  && resolvedRomajiAA  !== resolvedAlbumArtist) ffArgs.push('-metadata', `ALBUMARTISTSORT=${resolvedRomajiAA}`);
+      if (romajiAlbum       && romajiAlbum       !== album)               ffArgs.push('-metadata', `ALBUMSORT=${romajiAlbum}`);
       if (year)        ffArgs.push('-metadata', `date=${year}`);
       if (trackNum)    ffArgs.push('-metadata', `track=${totalTracks ? `${trackNum}/${totalTracks}` : String(trackNum)}`);
       if (totalTracks) ffArgs.push('-metadata', `totaltracks=${String(totalTracks)}`);