feat(flutter/library): artist + album detail screens

Headers carry placeholder play buttons (wired in Task 18) + room for
LikeButton (Task 16). Artist screen has a 2-column album grid; album
screen has a track list with mm:ss durations. Routes :id-parameterized
under the shell so the PlayerBar persists.

flutter_client/lib/shared/routing.dart

@@ -5,6 +5,8 @@ import 'package:go_router/go_router.dart';
 import '../auth/auth_provider.dart';
 import '../auth/login_screen.dart';
 import '../auth/server_url_screen.dart';
+import '../library/album_detail_screen.dart';
+import '../library/artist_detail_screen.dart';
 import '../library/home_screen.dart';
 import 'widgets/version_gate.dart';
 
@@ -32,6 +34,14 @@ GoRouter buildRouter(Ref ref) {
         builder: (ctx, state, child) => VersionGate(child: _ShellWithPlayerBar(child: child)),
         routes: [
           GoRoute(path: '/home', builder: (_, __) => const HomeScreen()),
+          GoRoute(
+            path: '/artists/:id',
+            builder: (_, s) => ArtistDetailScreen(id: s.pathParameters['id']!),
+          ),
+          GoRoute(
+            path: '/albums/:id',
+            builder: (_, s) => AlbumDetailScreen(id: s.pathParameters['id']!),
+          ),
         ],
       ),
     ],