thread_view: Add recent history entries & adjust empty state (#36625)

Release Notes:

- N/A

crates/agent_ui/src/ui.rs

@@ -2,7 +2,6 @@ mod agent_notification;
 mod burn_mode_tooltip;
 mod context_pill;
 mod end_trial_upsell;
-// mod new_thread_button;
 mod onboarding_modal;
 pub mod preview;
 
@@ -10,5 +9,4 @@ pub use agent_notification::*;
 pub use burn_mode_tooltip::*;
 pub use context_pill::*;
 pub use end_trial_upsell::*;
-// pub use new_thread_button::*;
 pub use onboarding_modal::*;