Improve collaboration panel empty styling (#65290)

## Summary

The empty-state styling looked different on the Git and Collab panels.
Changed the Collab panel's styling to match the Git panel.
Closes #65182 

old styling:
<img width="342" height="189" alt="Screenshot 2026-10-07 at 17 25 08"
src="https://github.com/user-attachments/assets/d12f21c6-0606-4e51-8e5f-a8acdb4386b0"
/>

new styling:
<img width="336" height="212" alt="Screenshot 2026-10-07 at 17 34 38"
src="https://github.com/user-attachments/assets/ea594c8d-920a-46ae-9ce4-904057d0c2d5"
/> <img width="334" height="166" alt="Screenshot 2026-10-07 at 18 00 42"
src="https://github.com/user-attachments/assets/d3240c5d-0a7b-493f-a8fa-359e1e44dd52"
/>


Git panel (unchanged, for reference):
<img width="318" height="178" alt="Screenshot 2026-10-07 at 17 41 46"
src="https://github.com/user-attachments/assets/0ee38fae-c0f3-4069-8299-3dc1606f153c"
/>

## Testing

Built and ran on macOS. Opened an empty window with an empty project,
compared the Git and Collab panels (screenshots above).

## Self-review

- [x] I've reviewed my diff for quality, security, reliability, and
performance.
- [x] UI changes follow the
[checklist](https://zed.dev/docs/development/ui-checklist).
- [ ] Tests cover the new or changed behavior. (N/A, visual-only change,
verified manually)

Release Notes:

- Fixed inconsistent empty-state styling between the Collab and Git
panels.

---------

Co-authored-by: arondubovszky <94438458+MeH762@users.noreply.github.com>

crates/collab_ui/src/collab_panel.rs

@@ -2732,14 +2732,15 @@ impl CollabPanel {
 
         v_flex()
             .p_4()
-            .gap_4()
+            .gap_1()
             .size_full()
             .text_center()
             .justify_center()
-            .child(Label::new(collab_blurb))
+            .items_center()
+            .child(Label::new(collab_blurb).color(Color::Muted))
             .child(
                 Button::new(button_id, button_label)
-                    .full_width()
+                    .label_size(LabelSize::Small)
                     .start_icon(Icon::new(button_icon).color(Color::Muted))
                     .style(ButtonStyle::Outlined)
                     .disabled(is_busy)