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/shared/routing.dart

@@ -8,6 +8,7 @@ import '../auth/server_url_screen.dart';
 import '../library/album_detail_screen.dart';
 import '../library/artist_detail_screen.dart';
 import '../library/home_screen.dart';
+import '../player/player_bar.dart';
 import 'widgets/version_gate.dart';
 
 GoRouter buildRouter(Ref ref) {
@@ -53,10 +54,10 @@ class _ShellWithPlayerBar extends StatelessWidget {
   final Widget child;
   @override
   Widget build(BuildContext context) {
-    // PlayerBar is wired in Task 18; this placeholder leaves the shell slot.
     return Column(
       children: [
         Expanded(child: child),
+        const PlayerBar(),
       ],
     );
   }