feat(save): persist booster inventory

lib/data/save_repository.dart

@@ -2,6 +2,7 @@ import 'dart:convert';
 
 import 'package:shared_preferences/shared_preferences.dart';
 
+import '../game/models/booster.dart';
 import 'streak.dart';
 
 class StageProgress {
@@ -51,6 +52,10 @@ class SaveRepository {
       _reviewRequested = (json['flags']
               as Map<String, dynamic>?)?['reviewRequested'] as bool? ??
           false;
+      final boosters = json['boosters'] as Map<String, dynamic>? ?? {};
+      for (final t in BoosterType.values) {
+        _boosters[t] = boosters[t.name] as int? ?? 0;
+      }
     }
   }
 
@@ -68,6 +73,9 @@ class SaveRepository {
   bool _soundEnabled = true;
   bool _musicEnabled = true;
   bool _reviewRequested = false;
+  final Map<BoosterType, int> _boosters = {
+    for (final t in BoosterType.values) t: 0,
+  };
 
   StreakState get streak => _streak;
   bool get tutorialDone => _tutorialDone;
@@ -109,6 +117,22 @@ class SaveRepository {
     return _flush();
   }
 
+  int boosterCount(BoosterType type) => _boosters[type] ?? 0;
+
+  Future<void> grantBooster(BoosterType type, [int n = 1]) {
+    _boosters[type] = (_boosters[type] ?? 0) + n;
+    return _flush();
+  }
+
+  /// Spends one booster. Returns false (and changes nothing) when none are left.
+  Future<bool> consumeBooster(BoosterType type) async {
+    final have = _boosters[type] ?? 0;
+    if (have <= 0) return false;
+    _boosters[type] = have - 1;
+    await _flush();
+    return true;
+  }
+
   Future<void> recordEndlessScore(int score) {
     if (score > _endlessBest) _endlessBest = score;
     return _flush();
@@ -185,6 +209,7 @@ class SaveRepository {
             'reviewRequested': _reviewRequested,
           },
           'endless': {'best': _endlessBest},
+          'boosters': {for (final t in BoosterType.values) t.name: _boosters[t]},
         }),
       );
 }