Add user-visible output for remote operations (#25849)

This PR adds toasts for reporting success and errors from remote git
operations. This PR also adds a focus handle to notifications, in
anticipation of making them keyboard accessible.

Release Notes:

- N/A

---------

Co-authored-by: julia <julia@zed.dev>

crates/zeta/src/completion_diff_element.rs

@@ -78,7 +78,7 @@ impl CompletionDiffElement {
             font_style: settings.buffer_font.style,
             ..Default::default()
         };
-        let element = StyledText::new(diff).with_highlights(&text_style, diff_highlights);
+        let element = StyledText::new(diff).with_default_highlights(&text_style, diff_highlights);
         let text_layout = element.layout().clone();
 
         CompletionDiffElement {