assistant2: Revise thread visual design (#23083)

This PR adjusts the design of the assistant 2 threads with the goal of
reducing visual busyness. My intention is to remove the amount of lines
and borders given it is a relatively tight space. It also refines the
"generating" floating container style, finally leveraging linear
gradients that were recently added to GPUI! Now, we only display headers
for "you" messages. Assistant responses will be rendered right in the
panel; not bounded by a card container.

<img width="800" alt="Screenshot 2025-01-14 at 7 08 39 PM"
src="https://github.com/user-attachments/assets/a8ffa780-0ef2-4d4b-ae19-3f02fd2d63a6"
/>

Release Notes:

- N/A