feat: add library sync status polling with automatic page reload when background sync completes

Add /api/library-status endpoint returning ready/updated_at/error from library_cache with thread-safe lock. Add pollLibrarySync() function to check library status every 5 seconds and reload page when updated_at timestamp changes. Track lastLibrarySync timestamp to detect background sync completion. Save active view to sessionStorage before reload to restore navigation state

app.py

@@ -1129,6 +1129,11 @@ def api_rescan():
 def scan_status():
     with job_lock: return jsonify(jobs)
 
+@app.get("/api/library-status")
+def library_status():
+    with library_cache_lock:
+        return jsonify({"ready": library_cache["ready"], "updated_at": library_cache["updated_at"], "error": library_cache["error"]})
+
 @app.get("/api/downloads")
 def downloads():
     if not SAB_URL or not SAB_API_KEY: