ui: Clean up PlatformTitlebar implementation (#9413)

This PR cleans up the implementation of the `PlatformTitlebar` component
to better match our conventions for building UI components.

Release Notes:

- N/A

crates/ui/src/components/stories.rs

@@ -10,6 +10,7 @@ mod label;
 mod list;
 mod list_header;
 mod list_item;
+mod platform_titlebar;
 mod tab;
 mod tab_bar;
 mod toggle_button;
@@ -26,6 +27,7 @@ pub use label::*;
 pub use list::*;
 pub use list_header::*;
 pub use list_item::*;
+pub use platform_titlebar::*;
 pub use tab::*;
 pub use tab_bar::*;
 pub use toggle_button::*;