UI refactor for menu items

[nsarrazin]

It's hard to distinguish between past chats and the menu items when the bar is full. I think a low hanging fruit would be to add:

• A horizontal separator between the two
• Icons for the menu tabs
• Maybe play around with the typography, increasing font weight on menu items for example.

[coyotte508]

See also https://github.com/huggingface/chat-ui/issues/143

[fredguth]

I also would move Theme inside Settings.

[gary149]

Closing, will probably do a V2 redesign at some point.