Add stop button cancelling current chat generation

As reported by one of our users on Discord:

> There's no "stop" button in the chat thread - I'm creating a post and was using a Reddit URL as a source - chat couldn't scrape the source - it correctly gave me a message that sometimes sources have anti-scraper, etc - but then continued on by searching the web - and I noticed there's no button to stop the thinking chain - so I started a new post (conversation)

We should think about adding it.