feat(flutter): playlists list + detail screens

- models/playlist.dart (Playlist, PlaylistTrack, PlaylistDetail)
- api/endpoints/playlists.dart (list with kind=user|system|all, get detail)
- playlists/playlists_provider.dart (Riverpod family providers)
- playlists/playlists_list_screen.dart (with system-variant pill)
- playlists/playlist_detail_screen.dart (header + Play button + rows)
- /playlists + /playlists/:id routes wired
- Home AppBar: queue_music icon next to Search

Tracks with track_id=null render greyed-out + struck-through (matches
web's PlaylistTrackRow behavior). Tap a row plays from that position;
top-level Play button plays the full playable subset from track 0.

flutter_client/lib/library/home_screen.dart

@@ -29,6 +29,11 @@ class HomeScreen extends ConsumerWidget {
         elevation: 0,
         title: Text('Minstrel', style: TextStyle(color: fs.parchment)),
         actions: [
+          IconButton(
+            icon: Icon(Icons.queue_music, color: fs.parchment),
+            tooltip: 'Playlists',
+            onPressed: () => context.push('/playlists'),
+          ),
           IconButton(
             icon: Icon(Icons.search, color: fs.parchment),
             tooltip: 'Search',