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/agent/Cargo.toml

@@ -84,7 +84,6 @@ ui.workspace = true
 ui_input.workspace = true
 util.workspace = true
 uuid.workspace = true
-vim_mode_setting.workspace = true
 workspace.workspace = true
 zed_actions.workspace = true
 workspace-hack.workspace = true