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/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);
+  });
+}