agent: Add button to continue iterating once all reviews are done (#28027)

This PR adds a button on the review tab empty state that toggles the
focus back to the agent panel so that users can keep iterating on the
thread that's active in the panel.

<img
src="https://github.com/user-attachments/assets/ace5cf93-8869-49bb-8106-e03a9e3c90f2"
width="700"/>

Release Notes:

- N/A

crates/icons/src/icons.rs

@@ -122,6 +122,7 @@ pub enum IconName {
     Font,
     FontSize,
     FontWeight,
+    ForwardArrow,
     GenericClose,
     GenericMaximize,
     GenericMinimize,