feat(ui): rewarded-ad grant for an empty booster

Tapping an empty booster opens the get-one dialog; confirming watches a
rewarded ad and, on earn, grants +1 of that booster and logs booster_granted
(source: ad). Covered by a widget test using an uninitialized AdService whose
showRewarded() resolves true.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/ui/screens/game_screen.dart

@@ -206,11 +206,10 @@ class _GameScreenState extends ConsumerState<GameScreen>
     );
   }
 
-  /// An empty booster offers to get one. The rewarded-ad grant is wired in
-  /// Task 15.
+  /// Task 15: an empty booster offers a rewarded ad; on reward, grant +1.
   Future<void> _offerBoosterAd(BoosterType type) async {
     final l10n = AppLocalizations.of(context)!;
-    await showDialog<bool>(
+    final confirmed = await showDialog<bool>(
       context: context,
       builder: (context) => AlertDialog(
         content: Text(l10n.boosterGetWithAd),
@@ -226,6 +225,14 @@ class _GameScreenState extends ConsumerState<GameScreen>
         ],
       ),
     );
+    if (confirmed != true) return;
+    final earned = await ref.read(adServiceProvider).showRewarded();
+    if (earned) {
+      await ref.read(boosterInventoryProvider.notifier).grant(type, 1);
+      ref
+          .read(analyticsProvider)
+          .boosterGranted(type: type.name, count: 1, source: 'ad');
+    }
   }
 
   @override

test/ui/game_screen_booster_test.dart

@@ -4,14 +4,16 @@
 // the board is laid out inside an Expanded/Center whose pixel geometry is not
 // deterministic in a widget test, so a pixel-accurate cell tap is unreliable.
 // Those paths are left to manual QA (see the agent report). What we CAN verify
-// deterministically is the inventory side-effect: shuffle applies immediately
-// and spends one booster.
+// deterministically is the inventory side-effects:
+//   - shuffle applies immediately and spends one booster
+//   - tapping an empty booster shows the ad dialog whose confirm grants +1
 import 'package:block_seasons/core/rng.dart';
 import 'package:block_seasons/data/save_repository.dart';
 import 'package:block_seasons/game/engine/piece_generator.dart';
 import 'package:block_seasons/game/models/booster.dart';
 import 'package:block_seasons/game/models/stage.dart';
 import 'package:block_seasons/l10n/gen/app_localizations.dart';
+import 'package:block_seasons/services/ad_service.dart';
 import 'package:block_seasons/state/providers.dart';
 import 'package:block_seasons/ui/screens/game_screen.dart';
 import 'package:flutter/material.dart';
@@ -35,6 +37,11 @@ StageConfig _stage() => StageConfig.fromJson({
       },
     });
 
+/// A real, uninitialized [AdService]: with no SDK loaded its [showRewarded]
+/// takes the "no ad available -> grant the reward" path and resolves true
+/// without touching the platform — exactly the rewarded-earn case we want.
+AdService _earnAd() => AdService(adsRemoved: () => true);
+
 Widget _wrap(ProviderContainer c) => UncontrolledProviderScope(
       container: c,
       child: MaterialApp(
@@ -49,11 +56,12 @@ Widget _wrap(ProviderContainer c) => UncontrolledProviderScope(
 void main() {
   TestWidgetsFlutterBinding.ensureInitialized();
 
-  Future<ProviderContainer> startedContainer() async {
+  Future<ProviderContainer> startedContainer({AdService? ad}) async {
     SharedPreferences.setMockInitialValues({});
     final repo = SaveRepository(await SharedPreferences.getInstance());
     final c = ProviderContainer(overrides: [
       saveRepositoryProvider.overrideWithValue(repo),
+      if (ad != null) adServiceProvider.overrideWithValue(ad),
     ]);
     c.read(gameSessionProvider.notifier).startStage(
           _stage(),
@@ -78,4 +86,27 @@ void main() {
     expect(c.read(boosterInventoryProvider)[BoosterType.shuffle], 0);
     c.dispose();
   });
+
+  testWidgets('tapping an empty booster offers an ad that grants +1',
+      (tester) async {
+    final c = await startedContainer(ad: _earnAd());
+    expect(c.read(boosterInventoryProvider)[BoosterType.hammer], 0);
+
+    await tester.pumpWidget(_wrap(c));
+    await tester.pump();
+
+    await tester.tap(find.byKey(const ValueKey('booster_hammer')));
+    await tester.pumpAndSettle();
+
+    // The dialog offers the "watch an ad to get one" action; confirm it.
+    final l10n = AppLocalizations.of(
+      tester.element(find.byType(GameScreen)),
+    )!;
+    await tester.tap(find.text(l10n.boosterGetWithAd).last);
+    await tester.pump();
+    await tester.pump();
+
+    expect(c.read(boosterInventoryProvider)[BoosterType.hammer], 1);
+    c.dispose();
+  });
 }