后端：

完善注册页面

backend/IM_API/Models/Group.cs

@@ -38,7 +38,7 @@ public partial class Group
     /// <summary>
     /// 群公告
     /// </summary>
-    public string? Announcement { get; set; }
+    public string Announcement { get; set; } = "暂无群公告，点击编辑添加。";
 
     /// <summary>
     /// 群聊创建时间
@@ -56,7 +56,6 @@ public partial class Group
     public DateTimeOffset LastUpdateTime { get; set; } = DateTime.UtcNow;
 
 
-    public virtual ICollection<GroupInvite> GroupInvites { get; set; } = new List<GroupInvite>();
 
     public virtual User GroupMasterNavigation { get; set; } = null!;