fix(library): restore missing async on _fetchLibraryData

Regression from 9f7045f dropped the async keyword, causing
"await isn't allowed in non-async function" build failure.

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

Author: pythoninthegrass

app/frontend/js/stores/library.js

@@ -210,7 +210,7 @@ export function createLibraryStore(Alpine) {
     /**
      * Fetch library data from backend API
      */
-    _fetchLibraryData() {
+    async _fetchLibraryData() {
       // Map frontend sort keys to backend column names
       const sortKeyMap = {
         default: 'album',