fix: UserProfile.phone 改为可空，消除非空强转崩溃点

- 跨端契约核对发现（功能清单「跨端核对发现」项）：服务端 phone 可返回 null
- 门禁：format 0 changed / analyze 0 issues / flutter test 30 passed

lib/features/auth/auth_models.dart

@@ -45,13 +45,14 @@ class UserProfile {
     return UserProfile(
       userId: json['userId'] as String,
       username: json['username'] as String,
-      phone: json['phone'] as String,
+      // 服务端可返回 null（历史数据或未来第三方注册），不得非空强转。
+      phone: json['phone'] as String?,
       createdAt: DateTime.parse(json['createdAt'] as String),
     );
   }
 
   final String userId;
   final String username;
-  final String phone;
+  final String? phone;
   final DateTime createdAt;
 }