Closes #23735

This PR fixes an issue where Zed shows a transparent title bar in
fullscreen mode on macOS instead of the default gray one.

When switching to fullscreen mode, we change the title bar appearance to
opaque. When exiting fullscreen mode, we check the existing
`appears_transparent` flag that we pass to gpui to decide whether to
change the title bar back to transparent or not.

Note: Regardless of the `appears_transparent` flag, gpui should always
show an opaque title bar in fullscreen mode to prevent a broken
appearance, as macOS always displays the title bar in fullscreen mode
upon mouse interaction.


https://github.com/user-attachments/assets/211fb185-239b-454e-ac7f-b93b25d33805

Release Notes:

- Fixed issue where Zed showed transparent titlebar in fullscreen mode
on macOS.