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/storybook/src/story_selector.rs

@@ -29,6 +29,7 @@ pub enum ComponentStory {
     ListHeader,
     ListItem,
     OverflowScroll,
+    PlatformTitlebar,
     Scroll,
     Tab,
     TabBar,
@@ -60,6 +61,7 @@ impl ComponentStory {
             Self::ListHeader => cx.new_view(|_| ui::ListHeaderStory).into(),
             Self::ListItem => cx.new_view(|_| ui::ListItemStory).into(),
             Self::OverflowScroll => cx.new_view(|_| crate::stories::OverflowScrollStory).into(),
+            Self::PlatformTitlebar => cx.new_view(|_| ui::PlatformTitlebarStory).into(),
             Self::Scroll => ScrollStory::view(cx).into(),
             Self::Text => TextStory::view(cx).into(),
             Self::Tab => cx.new_view(|_| ui::TabStory).into(),