git_ui: Clean up file history view design (#43941)

Mostly cleaning up the UI code. The UI looks fairly the same just a bit
more polished, with proper colors and spacing. Also added a scrollbar in
there. Next step would be to make it keyboard navigable.

<img width="500" height="1948" alt="Screenshot 2025-12-01 at 5  38@2x"
src="https://github.com/user-attachments/assets/c266b97c-4a79-4a0e-8e78-2f1ed1ba495f"
/>

Release Notes:

- N/A

crates/ui/src/components/avatar.rs

@@ -91,10 +91,16 @@ impl RenderOnce for Avatar {
                 self.image
                     .size(image_size)
                     .rounded_full()
-                    .bg(cx.theme().colors().ghost_element_background)
+                    .bg(cx.theme().colors().element_disabled)
                     .with_fallback(|| {
-                        Icon::new(IconName::Person)
-                            .color(Color::Muted)
+                        h_flex()
+                            .size_full()
+                            .justify_center()
+                            .child(
+                                Icon::new(IconName::Person)
+                                    .color(Color::Muted)
+                                    .size(IconSize::Small),
+                            )
                             .into_any_element()
                     }),
             )