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/features/settings/presentation/screens/coin_center_screen.dart

@@ -1,21 +1,35 @@
+import 'dart:io';
+
 import 'package:flutter/material.dart';
+import 'package:in_app_purchase/in_app_purchase.dart';
 
 import '../../../../app/di/injection.dart';
 import '../../../../core/auth/session_store.dart';
 import '../../../../core/logging/logger.dart';
+import '../../../../core/network/api_problem_mapper.dart';
 import '../../../../data/network/api_client.dart';
 import '../../../../data/storage/local_kv_store.dart';
 import '../../../../l10n/app_localizations.dart';
 import '../../../../shared/theme/app_color_palette.dart';
 import '../../../../shared/theme/design_tokens.dart';
+import '../../../../shared/widgets/toast/toast.dart';
+import '../../../../shared/widgets/toast/toast_type.dart';
+import '../../../payments/data/services/apple_iap_service.dart';
 import '../../../points/data/apis/points_api.dart';
 import '../../../points/data/models/package_info.dart';
 import '../widgets/settings_section_widgets.dart';
 
 class CoinCenterScreen extends StatefulWidget {
-  const CoinCenterScreen({super.key, required this.balance});
+  const CoinCenterScreen({
+    super.key,
+    required this.balance,
+    required this.userId,
+    required this.onBalanceChanged,
+  });
 
   final int balance;
+  final String userId;
+  final void Function(int newBalance) onBalanceChanged;
 
   @override
   State<CoinCenterScreen> createState() => _CoinCenterScreenState();
@@ -25,6 +39,7 @@ class _CoinCenterScreenState extends State<CoinCenterScreen> {
   final Logger _logger = getLogger('features.settings.coin_center_screen');
   List<PackageInfo>? _packages;
   bool _isLoading = true;
+  AppleIapService? _iapService;
 
   @override
   void initState() {
@@ -39,12 +54,26 @@ class _CoinCenterScreenState extends State<CoinCenterScreen> {
         baseUrl: appDependencies.backendUrl,
         tokenProvider: sessionStore.getToken,
       );
+
       final api = PointsApi(apiClient.rawDio);
       final result = await api.getPackages();
+
+      final service = AppleIapService(
+        apiClient: apiClient,
+        userId: widget.userId,
+      );
+      service.init();
+      service.addListener(_onPurchaseStateChanged);
+
+      if (await InAppPurchase.instance.isAvailable()) {
+        await service.loadStoreKitProducts(result.packages);
+      }
+
       if (mounted) {
         setState(() {
           _packages = result.packages;
           _isLoading = false;
+          _iapService = service;
         });
       }
     } catch (e, stackTrace) {
@@ -61,6 +90,60 @@ class _CoinCenterScreenState extends State<CoinCenterScreen> {
     }
   }
 
+  void _onPurchaseStateChanged() {
+    final service = _iapService;
+    if (service == null || !mounted) return;
+
+    final l10n = AppLocalizations.of(context)!;
+
+    switch (service.state) {
+      case PurchaseFlowState.success:
+        Toast.show(context, l10n.paymentSuccess, type: ToastType.success);
+        _refreshBalance();
+        service.resetState();
+        break;
+      case PurchaseFlowState.failed:
+        final apiProblem = service.lastApiProblem;
+        final error = apiProblem != null
+            ? mapApiProblemToMessage(apiProblem, l10n)
+            : (service.lastError ?? l10n.paymentVerifyFailed);
+        Toast.show(context, error, type: ToastType.error);
+        service.resetState();
+        break;
+      case PurchaseFlowState.purchasing:
+      case PurchaseFlowState.verifying:
+      case PurchaseFlowState.idle:
+        break;
+    }
+
+    setState(() {});
+  }
+
+  Future<void> _refreshBalance() async {
+    try {
+      final sessionStore = SessionStore(LocalKvStore());
+      final apiClient = ApiClient(
+        baseUrl: appDependencies.backendUrl,
+        tokenProvider: sessionStore.getToken,
+      );
+      final response = await apiClient.getJson('/api/v1/points/balance');
+      final newBalance = response['availableBalance'] as int;
+      widget.onBalanceChanged(newBalance);
+    } catch (e, stackTrace) {
+      _logger.warning(
+        message: 'Failed to refresh balance after purchase: $e',
+        extra: {'stackTrace': stackTrace.toString()},
+      );
+    }
+  }
+
+  @override
+  void dispose() {
+    _iapService?.removeListener(_onPurchaseStateChanged);
+    _iapService?.dispose();
+    super.dispose();
+  }
+
   @override
   Widget build(BuildContext context) {
     final l10n = AppLocalizations.of(context)!;
@@ -120,13 +203,13 @@ class _CoinCenterScreenState extends State<CoinCenterScreen> {
           ),
           const SizedBox(height: AppSpacing.xl),
           SectionLabel(text: l10n.settingsCoinRechargeSection),
-          ..._buildPackageCards(l10n),
+          ..._buildPackageCards(l10n, colors),
         ],
       ),
     );
   }
 
-  List<Widget> _buildPackageCards(AppLocalizations l10n) {
+  List<Widget> _buildPackageCards(AppLocalizations l10n, ColorScheme colors) {
     if (_isLoading) {
       return [
         const Padding(
@@ -140,22 +223,66 @@ class _CoinCenterScreenState extends State<CoinCenterScreen> {
       return [];
     }
 
+    final isBusy = _iapService?.state == PurchaseFlowState.purchasing ||
+        _iapService?.state == PurchaseFlowState.verifying;
+    final isPending = _iapService?.state == PurchaseFlowState.purchasing;
+
     return List.generate(_packages!.length, (index) {
       final pkg = _packages![index];
+      final storeKitProduct = _iapService?.getStoreKitProduct(pkg.appStoreProductId);
+      final isAvailable = storeKitProduct != null;
+
       return Column(
         children: [
           if (index > 0) const SizedBox(height: AppSpacing.md),
+          if (isPending && !isBusy)
+            Padding(
+              padding: const EdgeInsets.only(bottom: AppSpacing.sm),
+              child: Text(
+                l10n.paymentPending,
+                style: Theme.of(context).textTheme.bodySmall?.copyWith(
+                  color: colors.primary,
+                ),
+              ),
+            ),
           CoinPackageCard(
             title: _getPackageTitle(pkg, l10n),
-            price: pkg.priceDisplay,
+            price: _getDisplayPrice(pkg),
             amount: pkg.credits,
             badge: _getPackageBadge(pkg, l10n),
+            onPurchase: () => _handlePurchase(pkg),
+            isPurchasing: isBusy,
+            isAvailable: isAvailable,
+            unavailableMessage: isAvailable ? null : l10n.paymentProductUnavailable,
           ),
         ],
       );
     });
   }
 
+  String _getDisplayPrice(PackageInfo pkg) {
+    final product = _iapService?.getStoreKitProduct(pkg.appStoreProductId);
+    if (product != null) {
+      return product.price;
+    }
+    return pkg.priceDisplay;
+  }
+
+  Future<void> _handlePurchase(PackageInfo pkg) async {
+    final l10n = AppLocalizations.of(context)!;
+    if (_iapService == null) {
+      Toast.show(context, l10n.paymentVerifyFailed, type: ToastType.error);
+      return;
+    }
+
+    if (!Platform.isIOS) {
+      Toast.show(context, l10n.paymentVerifyFailed, type: ToastType.error);
+      return;
+    }
+
+    await _iapService!.purchase(pkg);
+  }
+
   String? _getPackageBadge(PackageInfo pkg, AppLocalizations l10n) {
     if (pkg.productCode == ProductCode.popularPack) {
       return l10n.settingsCoinPackPopularBadge;