WASM / Skia sample heads show white screen under WinUI 3

In #75, we added the ability to switch to WinUI 3 for all Uno projects. While this works and all platforms compile both inside and outside of Visual Studio, there's a lingering issue where Skia-based heads (WPF, Gtk) and WebAssembly show a blank white screen upon loading the app.

Upon inspecting the Visual Tree using the DevTools in WASM, it appears that the Arrange pass was skipped entirely, leaving all UI elements loaded but not displayed.