assistant2: Revise thread visual design

[danilo-leal]

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.

Release Notes:

• N/A

[Angelk90]

@danilo-leal : What do you think about this one, where it shows the type of assistant used:

Initially, the full text is displayed: claude-3-5-sonnet-20240620-fast-edit.
After a few seconds, the text gradually shrinks from left to right, showing only claude-3-5.
When the cursor hovers over the text, the animation reverses and the text expands from right to left, revealing the full text again.