thread_view: Adjust empty state and error displays (#36774)

Also changes the message editor placeholder depending on the agent.

Release Notes:

- N/A

---------

Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

crates/ui/src/components/callout.rs

@@ -132,6 +132,7 @@ impl RenderOnce for Callout {
 
         h_flex()
             .min_w_0()
+            .w_full()
             .p_2()
             .gap_2()
             .items_start()