fix(mobile-app): fix API response parsing for auto-create and wallet

- Extract 'data' field from API response before parsing
- Fix createAccount() to parse responseData['data']
- Fix getWalletInfo() to parse responseData['data']
- Resolves: type 'Null' is not a subtype of type 'int' in type cast

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

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

frontend/mobile-app/lib/core/services/account_service.dart

@@ -301,9 +301,9 @@ class AccountService {
       }
 
       debugPrint('$_tag createAccount() - 解析响应数据');
-      final result = CreateAccountResponse.fromJson(
-        response.data as Map<String, dynamic>,
-      );
+      final responseData = response.data as Map<String, dynamic>;
+      final data = responseData['data'] as Map<String, dynamic>;
+      final result = CreateAccountResponse.fromJson(data);
       debugPrint('$_tag createAccount() - 解析成功: $result');
 
       // 保存账号数据到安全存储
@@ -341,9 +341,9 @@ class AccountService {
       }
 
       debugPrint('$_tag getWalletInfo() - 解析响应数据');
-      final result = WalletInfoResponse.fromJson(
-        response.data as Map<String, dynamic>,
-      );
+      final responseData = response.data as Map<String, dynamic>;
+      final data = responseData['data'] as Map<String, dynamic>;
+      final result = WalletInfoResponse.fromJson(data);
       debugPrint('$_tag getWalletInfo() - 解析成功: $result');
 
       // 如果钱包已就绪，保存钱包地址和助记词