Cleanup chat model selector styles

[toolmantim]

This cleans up the chat model selector styles so it now has a keyboard focus outline, fixes some bugs with changing model with the keyboard, more closely matches VS Code styles (the webui toolkit is out of date), leaves space on the right for other buttons in the future (e.g. raw view), and includes the chat provider icon at the top of the chat.

Before	After

Test plan

• Start a new chat, select items
• Open an existing chat, verify disabled state

[toolmantim]

@abeatrix I wanted to also focus the chat input whenever you choose a new model, but I couldn't figure out how to do that. Any tips?

[abeatrix]

I wanted to also focus the chat input whenever you choose a new model, but I couldn't figure out how to do that. Any tips?

pushed some changes for this. Feel free to revert it if it doesn't work the way you want

[toolmantim]

That is perfect @abeatrix! Thank you!