feat(flutter/widgets): PlaylistCard + PlaylistPlaceholderCard

Mirrors the web equivalents at the same ~176dp width so the home
Playlists row keeps visual rhythm whether the slot is filled with a
real playlist or a placeholder.

PlaylistPlaceholderCard variants:
- building: spinner + "Building…"
- failed: warning + "Couldn't generate"
- seed-needed: muted icon + "Like more music"
- pending: muted icon + "Coming soon"

Used in the next commit's home Playlists row; otherwise unused this
slice.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter_client/test/playlists/widgets/playlist_card_test.dart

@@ -0,0 +1,58 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:minstrel/models/playlist.dart';
+import 'package:minstrel/playlists/widgets/playlist_card.dart';
+import 'package:minstrel/theme/theme_data.dart';
+
+const _userPlaylist = Playlist(
+  id: 'p1',
+  userId: 'u1',
+  name: 'Road trip',
+  description: '',
+  isPublic: false,
+  systemVariant: null,
+  trackCount: 12,
+  coverUrl: '',
+  ownerUsername: 'alice',
+  createdAt: '2026-05-01T00:00:00Z',
+  updatedAt: '2026-05-01T00:00:00Z',
+);
+
+const _forYou = Playlist(
+  id: 'p2',
+  userId: 'u1',
+  name: 'For You',
+  description: '',
+  isPublic: false,
+  systemVariant: 'for_you',
+  trackCount: 75,
+  coverUrl: '',
+  ownerUsername: 'alice',
+  createdAt: '2026-05-01T00:00:00Z',
+  updatedAt: '2026-05-01T00:00:00Z',
+);
+
+void main() {
+  testWidgets('renders name and no badge for user playlist', (tester) async {
+    await tester.pumpWidget(MaterialApp(
+      theme: buildThemeData(),
+      home: const Scaffold(
+        body: PlaylistCard(playlist: _userPlaylist),
+      ),
+    ));
+    expect(find.text('Road trip'), findsOneWidget);
+    expect(find.text('for you'), findsNothing);
+  });
+
+  testWidgets('renders system badge for system playlist', (tester) async {
+    await tester.pumpWidget(MaterialApp(
+      theme: buildThemeData(),
+      home: const Scaffold(
+        body: PlaylistCard(playlist: _forYou),
+      ),
+    ));
+    expect(find.text('For You'), findsOneWidget);
+    expect(find.text('for you'), findsOneWidget);
+  });
+}

flutter_client/test/playlists/widgets/playlist_placeholder_card_test.dart

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+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:minstrel/playlists/widgets/playlist_placeholder_card.dart';
+import 'package:minstrel/theme/theme_data.dart';
+
+void main() {
+  testWidgets('building variant renders spinner + label', (tester) async {
+    await tester.pumpWidget(MaterialApp(
+      theme: buildThemeData(),
+      home: const Scaffold(
+        body: PlaylistPlaceholderCard(label: 'For You', variant: 'building'),
+      ),
+    ));
+    expect(find.text('For You'), findsOneWidget);
+    expect(find.text('Building…'), findsOneWidget);
+    expect(find.byType(CircularProgressIndicator), findsOneWidget);
+  });
+
+  testWidgets('failed variant shows warning + copy', (tester) async {
+    await tester.pumpWidget(MaterialApp(
+      theme: buildThemeData(),
+      home: const Scaffold(
+        body: PlaylistPlaceholderCard(label: 'Songs like…', variant: 'failed'),
+      ),
+    ));
+    expect(find.text("Couldn't generate"), findsOneWidget);
+  });
+
+  testWidgets('seed-needed variant copy', (tester) async {
+    await tester.pumpWidget(MaterialApp(
+      theme: buildThemeData(),
+      home: const Scaffold(
+        body: PlaylistPlaceholderCard(label: 'Songs like…', variant: 'seed-needed'),
+      ),
+    ));
+    expect(find.text('Like more music'), findsOneWidget);
+  });
+
+  testWidgets('pending variant copy', (tester) async {
+    await tester.pumpWidget(MaterialApp(
+      theme: buildThemeData(),
+      home: const Scaffold(
+        body: PlaylistPlaceholderCard(label: 'For You', variant: 'pending'),
+      ),
+    ));
+    expect(find.text('Coming soon'), findsOneWidget);
+  });
+}