assistant: Adjust the toolbar design (#20101)

This PR's most relevant change is removing the three-dot menu dropdown
from the assistant toolbar. The "Regenerate Title" button is now only
visible on hover and it appears on the far right of the title input.

<img width="700" alt="Screenshot 2024-11-04 at 13 31 37"
src="https://github.com/user-attachments/assets/891703af-7985-4b16-bb5e-d852491abd6f">

Release Notes:

- N/A

crates/ui/src/components/icon.rs

@@ -234,6 +234,7 @@ pub enum IconName {
     Public,
     PullRequest,
     Quote,
+    RefreshTitle,
     Regex,
     ReplNeutral,
     Replace,