assistant2: Add empty state for new threads (#21542)

This PR adds an empty state for new threads in Assistant2:

<img width="1138" alt="Screenshot 2024-12-04 at 12 17 46 PM"
src="https://github.com/user-attachments/assets/ff7b4533-d3b8-4722-bd4b-43fac6d35a77">

This is mostly just a sketch in its current state.

Release Notes:

- N/A

crates/assistant2/src/assistant.rs

@@ -11,7 +11,13 @@ pub use crate::assistant_panel::AssistantPanel;
 
 actions!(
     assistant2,
-    [ToggleFocus, NewThread, ToggleModelSelector, Chat]
+    [
+        ToggleFocus,
+        NewThread,
+        ToggleModelSelector,
+        OpenHistory,
+        Chat
+    ]
 );
 
 const NAMESPACE: &str = "assistant2";