Add ability to stop an in-progress chat response

When a chat request is running, there is currently no way to stop it. If I notice a mistake in my prompt after submitting it, I have to wait for the response to finish before I can correct and resend it.

Add a Stop action while a response is being generated so the user can cancel the current request, update their prompt, and submit it again.

Expected behavior:
While a chat response is in progress, the send button should change to a stop/cancel button. Clicking it should immediately cancel the active request and allow the user to enter and submit a new prompt.

Look for the chat component that handles sending prompts and receiving streaming responses. Identify the button that changes state during a request. The stop action should cancel the active fetch or stream, reset the UI, and allow a new prompt. Check for existing API calls and state management for chat requests.

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