Bug: Editing or undoing a sent message doesn't work cleanly

When I send a message and immediately want to change it, there's no good way to do that.

If I hit stop right after sending, the app only lets me copy my text - I can't edit it in place. The edit flow seems like it's supposed to work, but it doesn't. So my only option is to retype the whole thing as a new message, which means the original bad prompt is still sitting in the context window eating tokens. Every false start or typo degrades the conversation.

Starting a new session over one bad message feels like overkill. I just want to fix what I said and move on.

What I've tried
- Editing the message inline: Doesn't work after hitting stop. I can copy but not edit.
- Sending a corrected follow-up: Works, but now I have two messages where I wanted one, and the context window carries both.
- Starting a new session: Solves the context pollution but throws away everything else in the conversation.

A few directions that would all solve this, not necessarily mutually exclusive:

- Fix the edit flow. If I stop a response and click to edit my message, let me actually edit it. And when I resubmit, replace the old tokens in context rather than appending - otherwise editing is just cosmetic.

- Add /undo or /rewind. One command that removes my last message and the assistant's response from the conversation and from the context window. Put my original text back in the composer so I can fix and resend.

未指定文件、测试或入口点。首先在桌面应用中复现停止后编辑的流程，然后跟踪已编辑或已撤销的消息在对话和上下文窗口中的表示方式。当停止的消息可以被编辑或回退、之前的消息和响应从上下文中移除，并且原始文本可在 composer 中使用时，即视为完成。

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