assistant2: Adjust empty state layout (#25745)

Going for a different, arguably simpler design for the Assistant 2 empty
state here. Also took the opportunity to adjust other elements like the
toolbar, message editor, and some items in the configuration page.

<img
src="https://github.com/user-attachments/assets/03fd1d48-a675-4eac-b694-bbe4eeaf06e9"
width="700px"/>

Release Notes:

- N/A

crates/ui/src/components/list/list_item.rs

@@ -38,6 +38,7 @@ pub struct ListItem {
     children: SmallVec<[AnyElement; 2]>,
     selectable: bool,
     outlined: bool,
+    rounded: bool,
     overflow_x: bool,
     focused: Option<bool>,
 }
@@ -63,6 +64,7 @@ impl ListItem {
             children: SmallVec::new(),
             selectable: true,
             outlined: false,
+            rounded: false,
             overflow_x: false,
             focused: None,
         }
@@ -147,6 +149,11 @@ impl ListItem {
         self
     }
 
+    pub fn rounded(mut self) -> Self {
+        self.rounded = true;
+        self
+    }
+
     pub fn overflow_x(mut self) -> Self {
         self.overflow_x = true;
         self
@@ -210,13 +217,13 @@ impl RenderOnce for ListItem {
                             })
                     })
             })
+            .when(self.rounded, |this| this.rounded_md())
             .child(
                 h_flex()
                     .id("inner_list_item")
                     .group("list_item")
                     .w_full()
                     .relative()
-                    .items_center()
                     .gap_1()
                     .px(DynamicSpacing::Base06.rems(cx))
                     .map(|this| match self.spacing {