add theme titlebar height and app-owned strip dragging

crates/theme/src/lib.rs

@@ -53,6 +53,8 @@ pub struct Theme {
     pub motion: Motion,
     /// Overlay scrollbar geometry.
     pub scrollbar: Scrollbar,
+    /// The height the window's titlebar strips draw at, per the platform.
+    pub titlebar_height: Pixels,
     /// The platform's native root font size; rem-based sizing scales with it.
     pub root_font_size: Pixels,
     pub sizes: SizeTable,
@@ -403,6 +405,7 @@ impl Theme {
             elevations: resolved.elevations,
             motion: resolved.motion,
             scrollbar: resolved.scrollbar,
+            titlebar_height: resolved.titlebar_height,
             root_font_size: resolved.root_font_size,
             sizes: resolved.sizes,
             accessibility: resolved.accessibility,