thread view: Refine the UI a bit (#36504)

Release Notes:

- N/A

---------

Co-authored-by: Agus Zubiaga <agus@zed.dev>

crates/icons/src/icons.rs

@@ -155,6 +155,7 @@ pub enum IconName {
     Maximize,
     Menu,
     MenuAlt,
+    MenuAltTemp,
     Mic,
     MicMute,
     Minimize,
@@ -245,6 +246,8 @@ pub enum IconName {
     Warning,
     WholeWord,
     XCircle,
+    XCircleFilled,
+    ZedAgent,
     ZedAssistant,
     ZedBurnMode,
     ZedBurnModeOn,