feat: 在向导页添加推荐码API验证

在用户点击"继续"按钮时,调用API验证推荐码是否有效:
- 推荐码不存在或已失效时显示错误提示
- 网络错误时显示友好提示
- 验证通过后才保存并跳转到注册页

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

frontend/mobile-app/lib/features/auth/presentation/pages/guide_page.dart

@@ -343,8 +343,25 @@ class _WelcomePageContentState extends ConsumerState<_WelcomePageContent> {
   Future<void> _saveReferralCodeAndProceed() async {
     if (!_canProceed) return;
 
-    // 推荐码必填，保存到本地存储
     final inviterCode = _referralCodeController.text.trim();
+
+    // 调用API验证推荐码是否有效
+    try {
+      final accountService = ref.read(accountServiceProvider);
+      final isValid = await accountService.verifyReferralCode(inviterCode);
+
+      if (!isValid) {
+        if (!mounted) return;
+        ScaffoldMessenger.of(context).showSnackBar(
+          const SnackBar(
+            content: Text('推荐码不存在或已失效'),
+            backgroundColor: Colors.redAccent,
+          ),
+        );
+        return;
+      }
+
+      // 验证通过，保存到本地存储
       final secureStorage = ref.read(secureStorageProvider);
       await secureStorage.write(
         key: StorageKeys.inviterReferralCode,
@@ -359,6 +376,16 @@ class _WelcomePageContentState extends ConsumerState<_WelcomePageContent> {
         RoutePaths.phoneRegister,
         extra: PhoneRegisterParams(inviterReferralCode: inviterCode),
       );
+    } catch (e) {
+      debugPrint('[GuidePage] 验证推荐码失败: $e');
+      if (!mounted) return;
+      ScaffoldMessenger.of(context).showSnackBar(
+        const SnackBar(
+          content: Text('网络错误，请稍后重试'),
+          backgroundColor: Colors.redAccent,
+        ),
+      );
+    }
   }
 
   /// 打开二维码扫描页面