copilot_ui: Fix sign-in window floating above all applications (#59657)

The Copilot code verification window was opened with
`WindowKind::PopUp`,
which causes the window to float above all other applications.
Changed to `WindowKind::Normal` so it stays scoped to Zed.

Fixes #51043

Release Notes:

- Fixed Copilot sign-in window floating above all other applications
instead of being scoped to Zed

crates/copilot_ui/src/sign_in.rs

@@ -63,7 +63,7 @@ fn open_copilot_code_verification_window(copilot: &Entity<Copilot>, window: &Win
     ));
     cx.open_window(
         WindowOptions {
-            kind: gpui::WindowKind::PopUp,
+            kind: gpui::WindowKind::Floating,
             window_bounds: Some(window_bounds),
             is_resizable: false,
             is_movable: true,