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/home_screen.dart

@@ -5,6 +5,7 @@ import 'package:go_router/go_router.dart';
 import '../models/album.dart';
 import '../models/artist.dart';
 import '../models/track.dart';
+import '../player/player_provider.dart';
 import '../theme/theme_extension.dart';
 import 'library_providers.dart';
 import 'widgets/album_card.dart';
@@ -66,7 +67,10 @@ class HomeScreen extends ConsumerWidget {
           for (final t in tracks)
             SizedBox(
               width: 280,
-              child: TrackRow(track: t, onTap: () { /* play wired in Task 18 */ }),
+              child: TrackRow(
+                track: t,
+                onTap: () => ref.read(playerActionsProvider).playTracks([t]),
+              ),
             ),
         ],
       );