feat(ui): presentational booster bar

lib/ui/widgets/booster_bar.dart

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+// lib/ui/widgets/booster_bar.dart
+import 'package:flutter/material.dart';
+
+import '../../game/models/booster.dart';
+
+class BoosterBar extends StatelessWidget {
+  const BoosterBar({super.key, required this.counts, required this.onTap});
+
+  final Map<BoosterType, int> counts;
+  final void Function(BoosterType) onTap;
+
+  static const _icons = {
+    BoosterType.hammer: Icons.gavel,
+    BoosterType.shuffle: Icons.shuffle,
+    BoosterType.lineBomb: Icons.clear_all,
+  };
+
+  @override
+  Widget build(BuildContext context) {
+    return Row(
+      mainAxisAlignment: MainAxisAlignment.spaceEvenly,
+      children: [
+        for (final t in BoosterType.values)
+          _BoosterButton(
+            key: ValueKey('booster_${t.name}'),
+            icon: _icons[t]!,
+            count: counts[t] ?? 0,
+            onTap: () => onTap(t),
+          ),
+      ],
+    );
+  }
+}
+
+class _BoosterButton extends StatelessWidget {
+  const _BoosterButton(
+      {super.key, required this.icon, required this.count, required this.onTap});
+  final IconData icon;
+  final int count;
+  final VoidCallback onTap;
+
+  @override
+  Widget build(BuildContext context) {
+    return InkWell(
+      onTap: onTap,
+      borderRadius: BorderRadius.circular(12),
+      child: Padding(
+        padding: const EdgeInsets.all(8),
+        child: Column(mainAxisSize: MainAxisSize.min, children: [
+          Icon(icon, size: 28),
+          const SizedBox(height: 2),
+          Text('$count'),
+        ]),
+      ),
+    );
+  }
+}

test/ui/booster_bar_test.dart

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+import 'package:block_seasons/game/models/booster.dart';
+import 'package:block_seasons/ui/widgets/booster_bar.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('renders three boosters with their counts', (tester) async {
+    BoosterType? tapped;
+    await tester.pumpWidget(MaterialApp(
+      home: Scaffold(
+        body: BoosterBar(
+          counts: const {
+            BoosterType.hammer: 3,
+            BoosterType.shuffle: 0,
+            BoosterType.lineBomb: 1,
+          },
+          onTap: (t) => tapped = t,
+        ),
+      ),
+    ));
+
+    expect(find.text('3'), findsOneWidget);
+    expect(find.text('1'), findsOneWidget);
+    await tester.tap(find.byKey(const ValueKey('booster_hammer')));
+    expect(tapped, BoosterType.hammer);
+  });
+}