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/album_detail_screen_test.dart

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+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

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+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);
+  });
+}