feat: 实现 iOS Apple Pay 内购支付功能

前端:
- 集成 in_app_purchase 插件，实现 IAP 支付流程
- 添加支付模块 (payments/) 处理产品获取、购买、验证
- 积分中心页面集成 Apple Pay 购买入口
- 设置页面重构: 关于/隐私/协议直接展示，删除 legal_center 子页面
- 修复欢迎引导页滚动检测阈值问题
- 修复解卦结果页 iOS 侧滑返回手势被阻止的问题
- 邀请码绑定按钮临时禁用（待后端实现）

后端:
- 新增 apple_iap_transactions 表记录交易
- 实现 Apple 服务器端验证 (App Store Server API)
- 支付成功后自动发放积分
- 支持 Sandbox/Production 环境切换
- 添加退款处理和交易状态机

协议:
- 更新积分流水协议，支持 purchase/refund 类型
- 新增 PAYMENT_* 错误码

apps/lib/l10n/app_localizations.dart

@@ -2432,6 +2432,42 @@ abstract class AppLocalizations {
   /// In zh, this message translates to:
   /// **'图片大小不能超过5MB'**
   String get feedbackImageTooLarge;
+
+  /// No description provided for @paymentSuccess.
+  ///
+  /// In zh, this message translates to:
+  /// **'购买成功'**
+  String get paymentSuccess;
+
+  /// No description provided for @paymentVerifyFailed.
+  ///
+  /// In zh, this message translates to:
+  /// **'购买验证失败，请稍后重试'**
+  String get paymentVerifyFailed;
+
+  /// No description provided for @paymentProductNotFound.
+  ///
+  /// In zh, this message translates to:
+  /// **'商品暂时不可用'**
+  String get paymentProductNotFound;
+
+  /// No description provided for @paymentStarterPackIneligible.
+  ///
+  /// In zh, this message translates to:
+  /// **'新手包每位用户仅限购买一次'**
+  String get paymentStarterPackIneligible;
+
+  /// No description provided for @paymentProductUnavailable.
+  ///
+  /// In zh, this message translates to:
+  /// **'商品暂时不可用'**
+  String get paymentProductUnavailable;
+
+  /// No description provided for @paymentPending.
+  ///
+  /// In zh, this message translates to:
+  /// **'Apple 正在处理中，请稍候'**
+  String get paymentPending;
 }
 
 class _AppLocalizationsDelegate