Add `Stop` button when triggering AI Chat

[tomsmith8]

Example from OpenAI:

[Antanasijevic]

@tomsmith8 Here are additional states for search and follow up question, including stop button.
One thing we may consider is making search field a bit more subtle by using neutral colors instead of blue highlight.
Please, check out these layouts and let me know what you think.

Prototype
https://www.figma.com/proto/YuEbi4x32Mm8ZQJdnrs4uc/Second-Brain-Final-Designs?node-id=11757-52937&t=BfTArXJM7LSbG5wQ-1&scaling=min-zoom&content-scaling=fixed&page-id=1238%3A3033&starting-point-node-id=11757%3A52937&show-proto-sidebar=1

Figma project
https://www.figma.com/design/YuEbi4x32Mm8ZQJdnrs4uc/Second-Brain-Final-Designs?node-id=11754-52381&t=YaYHyxv1KA8M8N14-1