fix(contract-signing): 添加 userRealName 字段到 Flutter ContractSigningTask

修复签名参照显示功能：将后端返回的 userRealName 字段添加到
Flutter 客户端的 ContractSigningTask 模型中，用于在签名
面板显示用户姓名供参照。

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

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

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

@@ -23,6 +23,7 @@ class ContractSigningTask {
   final double totalAmount;
   final String provinceName;
   final String cityName;
+  final String? userRealName; // 用户实名（用于签名参照）
   final DateTime expiresAt;
   final DateTime? scrolledToBottomAt;
   final DateTime? acknowledgedAt;
@@ -41,6 +42,7 @@ class ContractSigningTask {
     required this.totalAmount,
     required this.provinceName,
     required this.cityName,
+    this.userRealName,
     required this.expiresAt,
     this.scrolledToBottomAt,
     this.acknowledgedAt,
@@ -61,6 +63,7 @@ class ContractSigningTask {
       totalAmount: (json['totalAmount'] ?? 0).toDouble(),
       provinceName: json['provinceName'] ?? '',
       cityName: json['cityName'] ?? '',
+      userRealName: json['userRealName'],
       expiresAt: json['expiresAt'] != null
           ? DateTime.parse(json['expiresAt'])
           : DateTime.now().add(const Duration(hours: 24)),