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/widgets/track_row.dart

@@ -3,6 +3,7 @@ import 'package:flutter/material.dart';
 import '../../models/track.dart';
 import '../../shared/widgets/track_actions/track_actions_button.dart';
 import '../../theme/theme_extension.dart';
+import 'cached_indicator.dart';
 
 class TrackRow extends StatelessWidget {
   const TrackRow({
@@ -55,6 +56,7 @@ class TrackRow extends StatelessWidget {
               ),
             ]),
           ),
+          CachedIndicator(trackId: track.id),
           Text('$mins:$secs', style: TextStyle(color: fs.ash, fontSize: 12)),
           if (trailing != null)
             Padding(