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.

Non vengono indicati file, test o punti di ingresso. Inizia riproducendo il flusso di arresto e modifica nell’app desktop, quindi traccia il modo in cui i messaggi modificati o annullati vengono rappresentati nella conversazione e nella finestra di contesto. Il lavoro è completato quando un messaggio arrestato può essere modificato o riavvolto, il messaggio e la risposta precedenti vengono rimossi dal contesto e il testo originale è disponibile nel composer.

Scritto dal modello di indicizzazione a partire dal testo della issue.