agent: Adjust the thread generation design (#28193)

This PR simplifies the button to send a new message as well as the
"generation" display design.

Release Notes:

- N/A

---------

Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>

crates/icons/src/icons.rs

@@ -191,6 +191,7 @@ pub enum IconName {
     SearchCode,
     SearchSelection,
     SelectAll,
+    Send,
     Server,
     Settings,
     SettingsAlt,
@@ -212,6 +213,7 @@ pub enum IconName {
     Star,
     StarFilled,
     Stop,
+    StopFilled,
     Strikethrough,
     Supermaven,
     SupermavenDisabled,