feat(statistics): 게임 통계 시스템 추가

- GameStatistics 모델 (전투, 퀘스트, 아이템 통계)
- StatisticsStorage 영구 저장
- StatisticsDialog UI

lib/src/core/storage/statistics_storage.dart

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+import 'dart:convert';
+import 'dart:io';
+
+import 'package:askiineverdie/src/core/model/game_statistics.dart';
+import 'package:path_provider/path_provider.dart';
+
+/// 게임 통계 저장소 (Statistics Storage)
+///
+/// 누적 통계 데이터 저장/로드 관리
+class StatisticsStorage {
+  StatisticsStorage();
+
+  static const String _fileName = 'game_statistics.json';
+
+  Directory? _storageDir;
+
+  Future<Directory> _getStorageDir() async {
+    if (_storageDir != null) return _storageDir!;
+    _storageDir = await getApplicationSupportDirectory();
+    return _storageDir!;
+  }
+
+  File _getFile(Directory dir) {
+    return File('${dir.path}/$_fileName');
+  }
+
+  /// 누적 통계 로드
+  Future<CumulativeStatistics> loadCumulative() async {
+    try {
+      final dir = await _getStorageDir();
+      final file = _getFile(dir);
+
+      if (!await file.exists()) {
+        return CumulativeStatistics.empty();
+      }
+
+      final content = await file.readAsString();
+      final json = jsonDecode(content) as Map<String, dynamic>;
+
+      // cumulative 키가 있으면 해당 데이터 사용
+      if (json.containsKey('cumulative')) {
+        return CumulativeStatistics.fromJson(
+          json['cumulative'] as Map<String, dynamic>,
+        );
+      }
+
+      // 직접 CumulativeStatistics JSON인 경우
+      return CumulativeStatistics.fromJson(json);
+    } catch (e) {
+      // 파일이 없거나 손상된 경우 빈 통계 반환
+      return CumulativeStatistics.empty();
+    }
+  }
+
+  /// 누적 통계 저장
+  Future<bool> saveCumulative(CumulativeStatistics stats) async {
+    try {
+      final dir = await _getStorageDir();
+      final file = _getFile(dir);
+
+      final json = {
+        'version': 1,
+        'lastUpdated': DateTime.now().toIso8601String(),
+        'cumulative': stats.toJson(),
+      };
+
+      final content = const JsonEncoder.withIndent('  ').convert(json);
+      await file.writeAsString(content);
+      return true;
+    } catch (e) {
+      return false;
+    }
+  }
+
+  /// 세션 종료 시 누적 통계 업데이트
+  Future<bool> mergeSession(SessionStatistics session) async {
+    final current = await loadCumulative();
+    final updated = current.mergeSession(session);
+    return saveCumulative(updated);
+  }
+
+  /// 최고 레벨 업데이트
+  Future<bool> updateHighestLevel(int level) async {
+    final current = await loadCumulative();
+    if (level <= current.highestLevel) return true;
+    final updated = current.updateHighestLevel(level);
+    return saveCumulative(updated);
+  }
+
+  /// 최대 골드 업데이트
+  Future<bool> updateHighestGold(int gold) async {
+    final current = await loadCumulative();
+    if (gold <= current.highestGoldHeld) return true;
+    final updated = current.updateHighestGold(gold);
+    return saveCumulative(updated);
+  }
+
+  /// 새 게임 시작 기록
+  Future<bool> recordGameStart() async {
+    final current = await loadCumulative();
+    final updated = current.recordGameStart();
+    return saveCumulative(updated);
+  }
+
+  /// 게임 클리어 기록
+  Future<bool> recordGameComplete() async {
+    final current = await loadCumulative();
+    final updated = current.recordGameComplete();
+    return saveCumulative(updated);
+  }
+
+  /// 통계 초기화 (테스트용)
+  Future<bool> clear() async {
+    try {
+      final dir = await _getStorageDir();
+      final file = _getFile(dir);
+
+      if (await file.exists()) {
+        await file.delete();
+      }
+      return true;
+    } catch (e) {
+      return false;
+    }
+  }
+}