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/state.js

@@ -1,6 +1,6 @@
 // Shared mutable state — imported as { S } in every module
 export const S = {
-  cfg: { url: '', user: '', pass: '', lastfmKey: '', backendUrl: '', showPlayCount: false },
+  cfg: { url: '', user: '', pass: '', lastfmKey: '', backendUrl: '', showPlayCount: false, showRomaji: false },
 
   // Playback
   queue: [],