ui_prompt: Fix prompt dialog is hard to see on large screen (#35348)

Closes #18516

Release Notes:

- Improved visibility of prompt dialog on Linux by dimming the
background.

crates/ui_prompt/src/ui_prompt.rs

@@ -190,7 +190,11 @@ impl Render for ZedPromptRenderer {
                 }),
             ));
 
-        div().size_full().occlude().child(
+        div()
+            .size_full()
+            .occlude()
+            .bg(gpui::black().opacity(0.2))
+            .child(
                 div()
                     .size_full()
                     .absolute()