fix: auto-populate artist sort map on album page — no Artists page visit needed

ensureArtistSortMap() fetches getArtists once per session (cached) and
populates _artistSortMap automatically. Called at start of loadAlbumDetail
when showRomaji is on, so artist sort names are always available.

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

src/api.js

@@ -95,6 +95,7 @@ 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
+let _artistSortFetch  = null; // in-flight dedup for ensureArtistSortMap
 
 function _buildRomajiMap(idx) {
   const m  = new Map();
@@ -122,7 +123,22 @@ export async function fetchRomajiIndex() {
   return _romajiIndexFetch;
 }
 
-export function invalidateRomajiIndex() { _romajiIndex = null; _romajiMap = null; _artistRomajiMap = null; _artistSortMap = null; _albumRomajiMap = null; }
+export function invalidateRomajiIndex() { _romajiIndex = null; _romajiMap = null; _artistRomajiMap = null; _artistSortMap = null; _albumRomajiMap = null; _artistSortFetch = null; }
+
+// Ensures _artistSortMap is populated from getArtists (cached — one network call per session).
+// Safe to call from anywhere; deduplicates concurrent calls automatically.
+export async function ensureArtistSortMap() {
+  if (_artistSortMap) return;
+  if (_artistSortFetch) return _artistSortFetch;
+  _artistSortFetch = apiCached('getArtists')
+    .then(r => {
+      const all = (r?.artists?.index || []).flatMap(i => i.artist || []);
+      cacheArtistSortNames(all);
+      _artistSortFetch = null;
+    })
+    .catch(() => { _artistSortFetch = null; });
+  return _artistSortFetch;
+}
 
 // Populate _artistSortMap from Navidrome artist objects (OpenSubsonic sortName field).
 // Called whenever getArtists or getArtist data is loaded so dispArtistById can use it.