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.

No files, tests, or entry points are named. Start by reproducing the stop-then-edit flow in the desktop app, then trace how edited or undone messages are represented in the conversation and context window. Done means a stopped message can be edited or rewound, the prior message and response are removed from context, and the original text is available in the composer.

Written by the indexing model from the issue text.