feat(flutter): CachedIndicator + Download buttons + app startup wiring

CachedIndicator (lib/library/widgets/cached_indicator.dart) — small
download glyph rendered next to a track row when AudioCacheManager
reports the track as cached. FutureBuilder one-shot.

Wiring:
  - track_row.dart: render CachedIndicator before the duration label
  - playlist_detail: 'Download' OutlinedButton next to Play; pins all
    playable tracks with source: autoPlaylist + SnackBar feedback
  - album_detail: 'Download' IconButton in the header; same pin pattern
  - app.dart: now ConsumerStatefulWidget — initState fires the initial
    sync + activates the prefetcher provider

Together these complete the operator-facing surfaces of the offline
slice: visible cache state, explicit download trigger, automatic
sync + queue-ahead prefetch.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter_client/lib/app.dart

@@ -1,15 +1,37 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
+import 'cache/prefetcher.dart';
+import 'cache/sync_controller.dart';
 import 'shared/routing.dart';
 import 'theme/theme_data.dart';
 import 'theme/theme_mode_provider.dart';
 
-class MinstrelApp extends ConsumerWidget {
+class MinstrelApp extends ConsumerStatefulWidget {
   const MinstrelApp({super.key});
 
   @override
-  Widget build(BuildContext context, WidgetRef ref) {
+  ConsumerState<MinstrelApp> createState() => _MinstrelAppState();
+}
+
+class _MinstrelAppState extends ConsumerState<MinstrelApp> {
+  @override
+  void initState() {
+    super.initState();
+    // Activate offline-mode infrastructure once the first frame ships.
+    // SyncController.sync() is connectivity-aware (no-ops on no auth /
+    // no server URL), so calling it unconditionally is safe.
+    // Reading prefetcherProvider runs its constructor, which wires the
+    // queue + settings listeners.
+    WidgetsBinding.instance.addPostFrameCallback((_) {
+      // ignore: unawaited_futures
+      ref.read(syncControllerProvider.notifier).sync();
+      ref.read(prefetcherProvider);
+    });
+  }
+
+  @override
+  Widget build(BuildContext context) {
     final router = ref.watch(routerProvider);
     final mode = ref.watch(themeModeProvider).value ?? AppThemeMode.system;
     return MaterialApp.router(