fix: dispArtistById prefers Navidrome sortName over kuromoji translation

Added _artistSortMap populated from getArtists (artist listing) and
getArtist (artist detail) responses. dispArtistById now checks the
Navidrome sort tag first, falls back to kuromoji only when missing.

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

src/api.js

@@ -91,7 +91,8 @@ 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 _artistRomajiMap = null; // Map<artistId, artistRomaji> — kuromoji backend
+let _artistSortMap   = null; // Map<artistId, sortName>    — Navidrome sort tag (preferred)
 let _albumRomajiMap  = null; // Map<albumName(JP), albumRomaji> — for album cards without song ID
 let _romajiIndexFetch = null; // in-flight promise dedup
 
@@ -121,7 +122,16 @@ export async function fetchRomajiIndex() {
   return _romajiIndexFetch;
 }
 
-export function invalidateRomajiIndex() { _romajiIndex = null; _romajiMap = null; _artistRomajiMap = null; _albumRomajiMap = null; }
+export function invalidateRomajiIndex() { _romajiIndex = null; _romajiMap = null; _artistRomajiMap = null; _artistSortMap = null; _albumRomajiMap = null; }
+
+// Populate _artistSortMap from Navidrome artist objects (OpenSubsonic sortName field).
+// Called whenever getArtists or getArtist data is loaded so dispArtistById can use it.
+export function cacheArtistSortNames(artists) {
+  if (!_artistSortMap) _artistSortMap = new Map();
+  for (const a of artists) {
+    if (a.id && a.sortName && !_isJp(a.sortName)) _artistSortMap.set(String(a.id), a.sortName);
+  }
+}
 
 // ─── ROMAJI OVERRIDES ───
 // Stored server-side (shared across all clients). Cached in memory per session.
@@ -164,6 +174,8 @@ export function dispArtistById(id, name) {
   if (!S.cfg.showRomaji) return name || '';
   const ov = getRomajiOverrides();
   if (name && ov[name]) return ov[name];
+  const sort = _artistSortMap?.get(String(id));
+  if (sort) return sort;
   return _artistRomajiMap?.get(String(id)) || name || '';
 }
 export function dispAlbumName(name) {