用户暂离功能UI提示 || The user temporarily leaves the function UI prompt

第一次部署Tailchat，感觉比我们现在用的IM好很多，现在正在研究工单POST通过机器人广播给不同用户。感谢提供这样一个平台。
以下是我的一个小建议
感觉需要加一个用户暂时离开的功能。可以知道对方不会回复消息。
看了下之前的issues，精准已读确实有点重。
但比如 用户离开电脑操作 x分钟，让用户头像左下角新增一个离开的icon，或者头像置灰
这个功能感觉会很实用，实现起来对熟悉框架的人也很快。

It is the first time to deploy Tailchat, and it feels much better than the IM we use now. Now we are studying the POST of work orders to be broadcast to different users through robots. Thanks for providing such a platform.
Here is a little suggestion from me
I feel like I need to add a function for users to leave temporarily. You can know that the other party will not reply to the message.
After looking at the previous issues, the accurate read is indeed a bit heavy.
But for example, if the user leaves the computer for x minutes, a leaving icon is added to the lower left corner of the user's avatar, or the avatar is grayed out.
This function feels very practical and can be implemented quickly for people who are familiar with the framework.

No files, tests, or entry points are named in the issue. First clarify the timeout, manual behavior, and visual state for a temporarily away user, then locate the frontend presence and avatar code and define tests for the chosen behavior.