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/library/album_detail_screen.dart

@@ -0,0 +1,55 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import '../theme/theme_extension.dart';
+import 'library_providers.dart';
+import 'widgets/track_row.dart';
+
+class AlbumDetailScreen extends ConsumerWidget {
+  const AlbumDetailScreen({required this.id, super.key});
+  final String id;
+
+  @override
+  Widget build(BuildContext context, WidgetRef ref) {
+    final fs = Theme.of(context).extension<FabledSwordTheme>()!;
+    return Scaffold(
+      appBar: AppBar(),
+      backgroundColor: fs.obsidian,
+      body: ref.watch(albumProvider(id)).when(
+            error: (e, _) => Center(child: Text('$e', style: TextStyle(color: fs.error))),
+            loading: () => const Center(child: CircularProgressIndicator()),
+            data: (r) => ListView(children: [
+              Padding(
+                padding: const EdgeInsets.all(16),
+                child: Row(children: [
+                  Container(width: 96, height: 96, color: fs.slate),
+                  const SizedBox(width: 16),
+                  Expanded(child: Column(
+                    crossAxisAlignment: CrossAxisAlignment.start,
+                    children: [
+                      Text(r.album.title, style: TextStyle(
+                        color: fs.parchment,
+                        fontFamily: fs.display.fontFamily,
+                        fontSize: 22,
+                      )),
+                      Text(r.album.artistName, style: TextStyle(color: fs.ash)),
+                    ],
+                  )),
+                  Container(
+                    width: 48, height: 48,
+                    decoration: BoxDecoration(color: fs.accent, shape: BoxShape.circle),
+                    child: IconButton(
+                      icon: Icon(Icons.play_arrow, color: fs.parchment),
+                      onPressed: () { /* play wired in Task 18 */ },
+                    ),
+                  ),
+                  // LikeButton goes here in Task 16.
+                ]),
+              ),
+              for (final t in r.tracks)
+                TrackRow(track: t, onTap: () { /* play wired in Task 18 */ }),
+            ]),
+          ),
+    );
+  }
+}

flutter_client/lib/library/artist_detail_screen.dart

@@ -0,0 +1,82 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:go_router/go_router.dart';
+
+import '../models/album.dart';
+import '../theme/theme_extension.dart';
+import 'library_providers.dart';
+import 'widgets/album_card.dart';
+
+class ArtistDetailScreen extends ConsumerWidget {
+  const ArtistDetailScreen({required this.id, super.key});
+  final String id;
+
+  @override
+  Widget build(BuildContext context, WidgetRef ref) {
+    final fs = Theme.of(context).extension<FabledSwordTheme>()!;
+    final artist = ref.watch(artistProvider(id));
+    final albums = ref.watch(artistAlbumsProvider(id));
+
+    return Scaffold(
+      appBar: AppBar(),
+      backgroundColor: fs.obsidian,
+      body: artist.when(
+        error: (e, _) => Center(child: Text('$e', style: TextStyle(color: fs.error))),
+        loading: () => const Center(child: CircularProgressIndicator()),
+        data: (a) => ListView(children: [
+          Padding(
+            padding: const EdgeInsets.all(16),
+            child: Row(children: [
+              Container(
+                width: 96, height: 96,
+                decoration: BoxDecoration(color: fs.slate, shape: BoxShape.circle),
+              ),
+              const SizedBox(width: 16),
+              Expanded(
+                child: Text(
+                  a.name,
+                  style: TextStyle(
+                    color: fs.parchment,
+                    fontFamily: fs.display.fontFamily,
+                    fontSize: 24,
+                  ),
+                ),
+              ),
+              Container(
+                width: 48, height: 48,
+                decoration: BoxDecoration(color: fs.accent, shape: BoxShape.circle),
+                child: IconButton(
+                  icon: Icon(Icons.play_arrow, color: fs.parchment),
+                  onPressed: () { /* play wired in Task 18 */ },
+                ),
+              ),
+              // LikeButton goes here in Task 16.
+            ]),
+          ),
+          const Padding(
+            padding: EdgeInsets.fromLTRB(16, 16, 16, 8),
+            child: Text('Albums', style: TextStyle(fontSize: 16)),
+          ),
+          albums.when(
+            error: (e, _) => Center(child: Text('$e', style: TextStyle(color: fs.error))),
+            loading: () => const Padding(padding: EdgeInsets.all(16), child: Center(child: CircularProgressIndicator())),
+            data: (list) => GridView.builder(
+              shrinkWrap: true,
+              physics: const NeverScrollableScrollPhysics(),
+              padding: const EdgeInsets.all(8),
+              gridDelegate: const SliverGridDelegateWithFixedCrossAxisCount(
+                crossAxisCount: 2,
+                childAspectRatio: 0.8,
+              ),
+              itemCount: list.length,
+              itemBuilder: (_, i) {
+                final AlbumRef album = list[i];
+                return AlbumCard(album: album, onTap: () => context.push('/albums/${album.id}'));
+              },
+            ),
+          ),
+        ]),
+      ),
+    );
+  }
+}

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']!),
+          ),
         ],
       ),
     ],

flutter_client/test/library/album_detail_screen_test.dart

@@ -0,0 +1,28 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:minstrel/library/album_detail_screen.dart';
+import 'package:minstrel/library/library_providers.dart';
+import 'package:minstrel/models/album.dart';
+import 'package:minstrel/models/track.dart';
+import 'package:minstrel/theme/theme_data.dart';
+
+void main() {
+  testWidgets('renders album header + track list', (tester) async {
+    await tester.pumpWidget(ProviderScope(
+      overrides: [
+        albumProvider('al-1').overrideWith((ref) async => (
+              album: const AlbumRef(id: 'al-1', title: 'Drukqs', artistId: 'a-1', artistName: 'Aphex Twin'),
+              tracks: const [
+                TrackRef(id: 't-1', title: 'Avril 14th', albumId: 'al-1', artistId: 'a-1', durationSec: 121, trackNumber: 4),
+              ],
+            )),
+      ],
+      child: MaterialApp(theme: buildThemeData(), home: const AlbumDetailScreen(id: 'al-1')),
+    ));
+    await tester.pumpAndSettle();
+    expect(find.text('Drukqs'), findsOneWidget);
+    expect(find.text('Avril 14th'), findsOneWidget);
+  });
+}

flutter_client/test/library/artist_detail_screen_test.dart

@@ -0,0 +1,23 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:minstrel/library/artist_detail_screen.dart';
+import 'package:minstrel/library/library_providers.dart';
+import 'package:minstrel/models/artist.dart';
+import 'package:minstrel/theme/theme_data.dart';
+
+void main() {
+  testWidgets('renders artist name and Albums header', (tester) async {
+    await tester.pumpWidget(ProviderScope(
+      overrides: [
+        artistProvider('a-1').overrideWith((ref) async => const ArtistRef(id: 'a-1', name: 'Aphex Twin')),
+        artistAlbumsProvider('a-1').overrideWith((ref) async => const []),
+      ],
+      child: MaterialApp(theme: buildThemeData(), home: const ArtistDetailScreen(id: 'a-1')),
+    ));
+    await tester.pumpAndSettle();
+    expect(find.text('Aphex Twin'), findsOneWidget);
+    expect(find.text('Albums'), findsOneWidget);
+  });
+}