Flyout should have a drop shadow

[chucker]

Describe the bug

I thought #1472 would resolve this, but the standard behavior of Flyout is still to lack a shadow. For example, in the Gallery app:

Contrast a similar Flyout from the WinUI 3 Gallery app:

To Reproduce

Take the Gallery's FlyoutPage; it exhibits the issue.

Expected behavior

There should be a drop shadow, similar to that in menus.

Screenshots

No response

OS version

Windows 11 25H2 26220.8340

.NET version

.NET 10

WPF-UI NuGet version

main at 829b019

Additional context

No response