fix: 修正推荐码验证的返回值类型处理

verifyReferralCode()返回Map<String, dynamic>,需要提取valid字段
同时从响应中获取message字段用于错误提示

🤖 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

@@ -348,13 +348,15 @@ class _WelcomePageContentState extends ConsumerState<_WelcomePageContent> {
     // 调用API验证推荐码是否有效
     try {
       final accountService = ref.read(accountServiceProvider);
-      final isValid = await accountService.verifyReferralCode(inviterCode);
+      final result = await accountService.verifyReferralCode(inviterCode);
 
+      final isValid = result['valid'] as bool? ?? false;
       if (!isValid) {
         if (!mounted) return;
+        final message = result['message'] as String? ?? '推荐码不存在或已失效';
         ScaffoldMessenger.of(context).showSnackBar(
-          const SnackBar(
-            content: Text('推荐码不存在或已失效'),
+          SnackBar(
+            content: Text(message),
             backgroundColor: Colors.redAccent,
           ),
         );