feat: romaji artist names, Apple Music autofill, romaji index invalidation

- Artist grid and detail header now show romaji names when toggle is on.
  Builds a Map<artistId, artistRomaji> from the existing song romaji index
  so no extra API call is needed.

- Apple Music autofill button added to edit modal next to MusicBrainz.
  Reuses the existing lookupItunesTrackMeta backend function via a new
  GET /api/itunes/lookup endpoint — fills album, year, genre, track#,
  and cover art.

- Romaji index is invalidated after every successful download so newly
  added songs appear with correct sort tags without requiring a page reload.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/api.js

@@ -53,11 +53,17 @@ export async function ndAuth() {
 // Fetched once per session from the backend, cached in memory.
 let _romajiIndex = null;
 let _romajiMap   = null; // Map<songId, { titleRomaji, artistRomaji, albumRomaji }>
+let _artistRomajiMap = null; // Map<artistId, artistRomaji>
 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));
+  const m  = new Map();
+  const am = new Map();
+  if (Array.isArray(idx)) idx.forEach(e => {
+    m.set(String(e.id), e);
+    if (e.artistId && e.artistRomaji) am.set(String(e.artistId), e.artistRomaji);
+  });
+  _artistRomajiMap = am;
   return m;
 }
 
@@ -73,7 +79,12 @@ export async function fetchRomajiIndex() {
   return _romajiIndexFetch;
 }
 
-export function invalidateRomajiIndex() { _romajiIndex = null; _romajiMap = null; }
+export function invalidateRomajiIndex() { _romajiIndex = null; _romajiMap = null; _artistRomajiMap = null; }
+
+export function dispArtistById(id, name) {
+  if (!S.cfg.showRomaji || !_artistRomajiMap) return name || '';
+  return _artistRomajiMap.get(String(id)) || name || '';
+}
 
 // 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; }