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/assistant2/src/message_editor.rs

@@ -314,7 +314,7 @@ impl Render for MessageEditor {
             .child(self.context_strip.clone())
             .child(
                 v_flex()
-                    .gap_4()
+                    .gap_5()
                     .child({
                         let settings = ThemeSettings::get_global(cx);
                         let text_style = TextStyle {