assistant2: Add tiny design improvements (#27399)

Really small stuff, like adding icons and adjusting colors.

Release Notes:

- N/A

crates/ui/src/components/icon.rs

@@ -140,6 +140,7 @@ pub enum IconName {
     ArrowDownFromLine,
     ArrowLeft,
     ArrowRight,
+    ArrowRightLeft,
     ArrowUp,
     ArrowUpFromLine,
     ArrowUpRight,
@@ -170,6 +171,7 @@ pub enum IconName {
     Circle,
     Close,
     Code,
+    Cog,
     Command,
     Context,
     Control,