feat(flutter/player): mini PlayerBar in shell + tap-to-play wiring

PlayerBar reads playbackState + mediaItem streams; hidden when no
queue. Home track-row tap, album play button, album track tap, artist
play button (shuffle) all route to playerActions.playTracks. Token +
baseUrl forwarded to the audio handler before each new queue.

flutter_client/lib/library/artist_detail_screen.dart

@@ -5,6 +5,7 @@ import 'package:go_router/go_router.dart';
 import '../api/endpoints/likes.dart';
 import '../likes/like_button.dart';
 import '../models/album.dart';
+import '../player/player_provider.dart';
 import '../theme/theme_extension.dart';
 import 'library_providers.dart';
 import 'widgets/album_card.dart';
@@ -49,7 +50,12 @@ class ArtistDetailScreen extends ConsumerWidget {
                 decoration: BoxDecoration(color: fs.accent, shape: BoxShape.circle),
                 child: IconButton(
                   icon: Icon(Icons.play_arrow, color: fs.parchment),
-                  onPressed: () { /* play wired in Task 18 */ },
+                  onPressed: () async {
+                    final tracks = await ref.read(artistTracksProvider(id).future);
+                    if (tracks.isEmpty) return;
+                    final shuffled = [...tracks]..shuffle();
+                    ref.read(playerActionsProvider).playTracks(shuffled);
+                  },
                 ),
               ),
               LikeButton(kind: LikeKind.artist, id: a.id, size: 28),