feat: 主页账号备份，并修好 B50 预览缩不小

账号备份（AccountBackupCard + AccountBackupService）：
- 并发读 GetUserData/Extend/Option/Rating/Charge/Activity/MissionData +
  Character + Map（分页）+ KaleidxScope + 各 itemKind 的 GetUserItemApi，
  汇成一个 JSON 字符串可复制到剪贴板。舞里程与总游玩数据本就在
  userData 的 point/totalPoint/playCount 里。
- itemKind 15（KaleidxScopeKey）不列进回读清单：客户端 ExportUserItems
  从不输出它，下行也没有读它的路径，钥匙实际是
  userKaleidxScopeList[].isKeyFound。
- 全程只发 GetUser*Api，不碰 UpsertUserAllApi。
- 逐段容错：私服没实现某个接口时其余照样备出，失败段单独列出——
  「读到但是空的」和「没读到」在 userItemList 里按 kind 分开存，可区分。
- 取数与拼装分开，buildBackupDocument/encode 部分是纯函数，单测覆盖。

B50 预览：原因是 InteractiveViewer(constrained:false) 给孩子无界约束，
FittedBox(fit: fitWidth) 在无界下根本不缩放，海报按 1762x1850 原尺寸铺开；
而 minScale 又算成 1.0，于是既巨大又缩不动。改为给一个 contain 尺寸的
有限盒子让 FittedBox 真正缩放，minScale 固定 1.0（这个版本没有
initialScale 可用）。RepaintBoundary 留在 FittedBox 内侧，绘制尺寸仍是
1762x1850，导出分辨率不受影响，并加了断言守住这点。

test/account_backup_test.dart

@@ -0,0 +1,93 @@
+import 'dart:convert';
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:ichino/services/account_backup_service.dart';
+
+void main() {
+  group('账号备份文档', () {
+    final sections = <String, Object?>{
+      'userData': {
+        'userData': {'point': 123456, 'totalPoint': 999999, 'playCount': 1234},
+      },
+      'userCharacterList': [
+        {'characterId': 101, 'level': 999999, 'awakening': 0, 'useCount': 7},
+      ],
+      'userMapList': [
+        {'mapId': 550002, 'distance': 999999999, 'isComplete': true},
+      ],
+      'userItemList': {
+        '5': [
+          {'itemKind': 5, 'itemId': 11820, 'stock': 1, 'isValid': true},
+        ],
+        '6': const <Object?>[],
+      },
+    };
+
+    test('带格式版本、userId 与 exportedAt，各段挂在 data 下', () {
+      final doc = buildBackupDocument(
+        userId: 42,
+        sections: sections,
+        exportedAt: DateTime.utc(2026, 10, 2, 3, 4, 5),
+      );
+
+      expect(doc['backupFormat'], AccountBackup.formatVersion);
+      expect(doc['userId'], 42);
+      expect(doc['exportedAt'], '2026-10-02T03:04:05.000Z');
+      expect(doc['data'], sections);
+    });
+
+    test('encode 出来的是能原样解回来的 JSON 字符串', () {
+      final backup = AccountBackup(
+        document: buildBackupDocument(userId: 42, sections: sections),
+        failedSections: const [],
+      );
+
+      final text = backup.encode();
+      expect(text, isA<String>());
+      expect(text.contains('11820'), isTrue);
+
+      final back = jsonDecode(text) as Map<String, dynamic>;
+      final data = back['data'] as Map<String, dynamic>;
+      final userData =
+          (data['userData'] as Map<String, dynamic>)['userData']
+              as Map<String, dynamic>;
+      expect(userData['point'], 123456);
+
+      final items = data['userItemList'] as Map<String, dynamic>;
+      expect(items['5'], hasLength(1));
+      // 空的那一类要留着：「读到但是空的」和「没读到」不是一回事。
+      expect(items.containsKey('6'), isTrue);
+      expect(items['6'], isEmpty);
+    });
+
+    test('单段失败不算整体失败，但会记下来并让 isComplete 变 false', () {
+      final backup = AccountBackup(
+        document: buildBackupDocument(userId: 7, sections: sections),
+        failedSections: const ['userMaps: HTTP 500', 'userItemList/12: 超时'],
+      );
+
+      expect(backup.isComplete, isFalse);
+      expect(backup.failedSections, hasLength(2));
+      // 失败信息不进 data，备份正文仍然可用。
+      expect(
+        (jsonDecode(backup.encode()) as Map)['data'].containsKey('userMaps'),
+        isFalse,
+      );
+    });
+
+    test('byteLength 与实际编码长度一致', () {
+      final backup = AccountBackup(
+        document: buildBackupDocument(userId: 7, sections: sections),
+        failedSections: const [],
+      );
+
+      expect(backup.byteLength, utf8.encode(backup.encode()).length);
+    });
+
+    test('备份要覆盖用户点名的那几类数据', () {
+      expect(kBackupItemKinds, containsAll([1, 2, 3, 5, 6, 7, 10, 11, 12]));
+      // 15 = KaleidxScopeKey，客户端从不读它，不该出现在回读清单里。
+      expect(kBackupItemKinds, isNot(contains(15)));
+    });
+  });
+}