assistant2: Add stray visual adjustments (#22111)

Mostly minor tweaks to make it closer to the prototype. More to come.

| With message | Empty state |
|--------|--------|
| <img width="1420" alt="Screenshot 2024-12-16 at 18 59 40"
src="https://github.com/user-attachments/assets/5df791bf-577a-4f01-9045-80568604099f"
/> | <img width="1420" alt="Screenshot 2024-12-16 at 18 59 33"
src="https://github.com/user-attachments/assets/adbf1673-3040-4b2b-8d65-f8b38a83c1d0"
/> |

Release Notes:

- N/A

crates/ui/src/components/icon.rs

@@ -218,6 +218,7 @@ pub enum IconName {
     Maximize,
     Menu,
     MessageBubbles,
+    MessageCircle,
     Mic,
     MicMute,
     Microscope,