[P1] Add a stop button to interrupt generation

[jasonthane]

No description provided.

[jonryser]

Convert the submit button to a "stop" button while the Ai is generating.
The "trashcan" button will remain while generating.
If the "stop" button is pressed, Ai generation terminates.
If the "trashcan" button is clicked, Ai generation terminates (if generating), a confirmation dialog makes it clear that the user is deleting the history.