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/library/album_detail_screen.dart

@@ -2,6 +2,8 @@ import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import '../api/endpoints/likes.dart';
+import '../cache/audio_cache_manager.dart';
+import '../cache/db.dart';
 import '../likes/like_button.dart';
 import '../player/player_provider.dart';
 import '../theme/theme_extension.dart';
@@ -38,6 +40,21 @@ class AlbumDetailScreen extends ConsumerWidget {
                       Text(r.album.artistName, style: TextStyle(color: fs.ash)),
                     ],
                   )),
+                  IconButton(
+                    key: const Key('download_album_button'),
+                    icon: Icon(Icons.download, color: fs.ash),
+                    tooltip: 'Download album',
+                    onPressed: () {
+                      final mgr = ref.read(audioCacheManagerProvider);
+                      for (final t in r.tracks) {
+                        // ignore: unawaited_futures
+                        mgr.pin(t.id, source: CacheSource.autoPlaylist);
+                      }
+                      ScaffoldMessenger.of(context).showSnackBar(
+                        SnackBar(content: Text('Downloading ${r.tracks.length} tracks…')),
+                      );
+                    },
+                  ),
                   Container(
                     width: 48, height: 48,
                     decoration: BoxDecoration(color: fs.accent, shape: BoxShape.circle),