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/assistant2/src/ui/context_pill.rs

@@ -29,9 +29,12 @@ impl RenderOnce for ContextPill {
     fn render(self, cx: &mut WindowContext) -> impl IntoElement {
         h_flex()
             .gap_1()
-            .px_1()
+            .pl_1p5()
+            .pr_0p5()
+            .pb(px(1.))
             .border_1()
-            .border_color(cx.theme().colors().border)
+            .border_color(cx.theme().colors().border.opacity(0.5))
+            .bg(cx.theme().colors().element_background)
             .rounded_md()
             .child(Label::new(self.context.name.clone()).size(LabelSize::Small))
             .when_some(self.on_remove, |parent, on_remove| {