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/playlists/playlist_detail_screen.dart

@@ -2,6 +2,8 @@ import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:go_router/go_router.dart';
 
+import '../cache/audio_cache_manager.dart';
+import '../cache/db.dart';
 import '../models/playlist.dart';
 import '../models/track.dart';
 import '../player/player_provider.dart';
@@ -138,7 +140,26 @@ class _Header extends ConsumerWidget {
             style: TextStyle(color: fs.ash, fontSize: 12),
           ),
           const Spacer(),
-          if (playable.isNotEmpty)
+          if (playable.isNotEmpty) ...[
+            OutlinedButton.icon(
+              key: const Key('download_playlist_button'),
+              onPressed: () {
+                final mgr = ref.read(audioCacheManagerProvider);
+                for (final t in playable) {
+                  final id = t.trackId ?? '';
+                  if (id.isEmpty) continue;
+                  // Fire-and-forget; downloads happen in background.
+                  // ignore: unawaited_futures
+                  mgr.pin(id, source: CacheSource.autoPlaylist);
+                }
+                ScaffoldMessenger.of(context).showSnackBar(
+                  SnackBar(content: Text('Downloading ${playable.length} tracks…')),
+                );
+              },
+              icon: const Icon(Icons.download, size: 16),
+              label: const Text('Download'),
+            ),
+            const SizedBox(width: 8),
             FilledButton.icon(
               onPressed: () {
                 final refs = playable.map(_toTrackRef).toList(growable: false);
@@ -151,6 +172,7 @@ class _Header extends ConsumerWidget {
                 foregroundColor: fs.parchment,
               ),
             ),
+          ],
         ]),
       ]),
     );