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/home/presentation/screens/home_screen.dart

@@ -27,6 +27,7 @@ class HomeScreen extends StatefulWidget {
   const HomeScreen({
     super.key,
     required this.account,
+    required this.userId,
     required this.sessionStore,
     required this.currentLocale,
     required this.profileSettings,
@@ -43,9 +44,11 @@ class HomeScreen extends StatefulWidget {
     required this.onDeleteHistorySession,
     required this.onLogout,
     required this.onDeleteAccount,
+    required this.onBalanceChanged,
   });
 
   final String account;
+  final String userId;
   final SessionStore sessionStore;
   final Locale currentLocale;
   final ProfileSettingsV1 profileSettings;
@@ -56,15 +59,16 @@ class HomeScreen extends StatefulWidget {
   final NotificationRepository notificationRepository;
   final Future<void> Function(String languageTag) onLocaleChanged;
   final Future<void> Function(ProfileSettingsV1 settings)
-  onProfileSettingsChanged;
+      onProfileSettingsChanged;
   final Future<ProfileSettingsV1> Function(ProfileSettingsV1 updated)
-  onSaveProfile;
+      onSaveProfile;
   final Future<ProfileSettingsV1> Function(String filePath) onUploadAvatar;
   final Future<void> Function(DivinationResultData result)
-  onDivinationCompleted;
+      onDivinationCompleted;
   final Future<void> Function(String threadId) onDeleteHistorySession;
   final Future<void> Function() onLogout;
   final Future<void> Function() onDeleteAccount;
+  final void Function(int newBalance) onBalanceChanged;
 
   @override
   State<HomeScreen> createState() => _HomeScreenState();
@@ -132,6 +136,7 @@ class _HomeScreenState extends State<HomeScreen> {
           ),
           _ProfileTab(
             account: widget.account,
+            userId: widget.userId,
             settings: widget.profileSettings,
             coinBalance: widget.coinBalance,
             inviteRepository: _inviteRepository,
@@ -142,6 +147,7 @@ class _HomeScreenState extends State<HomeScreen> {
             onUploadAvatar: widget.onUploadAvatar,
             onLogout: widget.onLogout,
             onDeleteAccount: widget.onDeleteAccount,
+            onBalanceChanged: widget.onBalanceChanged,
           ),
         ],
       ),
@@ -561,6 +567,7 @@ class _DivinationHistoryScreenState extends State<DivinationHistoryScreen> {
 class _ProfileTab extends StatelessWidget {
   const _ProfileTab({
     required this.account,
+    required this.userId,
     required this.settings,
     required this.coinBalance,
     required this.inviteRepository,
@@ -571,9 +578,11 @@ class _ProfileTab extends StatelessWidget {
     required this.onUploadAvatar,
     required this.onLogout,
     required this.onDeleteAccount,
+    required this.onBalanceChanged,
   });
 
   final String account;
+  final String userId;
   final ProfileSettingsV1 settings;
   final int coinBalance;
   final InviteRepository inviteRepository;
@@ -581,15 +590,17 @@ class _ProfileTab extends StatelessWidget {
   final Future<void> Function(String languageTag) onLocaleChanged;
   final Future<void> Function(ProfileSettingsV1 settings) onSettingsChanged;
   final Future<ProfileSettingsV1> Function(ProfileSettingsV1 updated)
-  onSaveProfile;
+      onSaveProfile;
   final Future<ProfileSettingsV1> Function(String filePath) onUploadAvatar;
   final Future<void> Function() onLogout;
   final Future<void> Function() onDeleteAccount;
+  final void Function(int newBalance) onBalanceChanged;
 
   @override
   Widget build(BuildContext context) {
     return SettingsScreen(
       account: account,
+      userId: userId,
       settings: settings,
       coinBalance: coinBalance,
       inviteRepository: inviteRepository,
@@ -600,6 +611,7 @@ class _ProfileTab extends StatelessWidget {
       onUploadAvatar: onUploadAvatar,
       onLogout: onLogout,
       onDeleteAccount: onDeleteAccount,
+      onBalanceChanged: onBalanceChanged,
     );
   }
 }
@@ -703,14 +715,23 @@ class _WelcomeDialog extends StatefulWidget {
 class _WelcomeDialogState extends State<_WelcomeDialog> {
   final ScrollController _scrollController = ScrollController();
   bool _hasScrolledToBottom = false;
+  bool _hasCheckedInitialScroll = false;
 
   @override
   void initState() {
     super.initState();
     _scrollController.addListener(_handleScroll);
-    WidgetsBinding.instance.addPostFrameCallback((_) {
-      _syncScrollState();
-    });
+  }
+
+  @override
+  void didChangeDependencies() {
+    super.didChangeDependencies();
+    if (!_hasCheckedInitialScroll) {
+      _hasCheckedInitialScroll = true;
+      WidgetsBinding.instance.addPostFrameCallback((_) {
+        _syncScrollState();
+      });
+    }
   }
 
   @override
@@ -730,7 +751,7 @@ class _WelcomeDialogState extends State<_WelcomeDialog> {
     }
     final max = _scrollController.position.maxScrollExtent;
     final current = _scrollController.offset;
-    final canReadAll = max <= AppSpacing.xs || current >= max - AppSpacing.md;
+    final canReadAll = max <= 50.0 || current >= max - AppSpacing.md;
     if (_hasScrolledToBottom == canReadAll) {
       return;
     }