Fix issues in mica.md

Restore missing '### Use Mica with WinUI for UWP' subsection that was
accidentally removed in a prior commit, leaving a broken cross-reference:
line 125 states UWP support exists but provided no how-to link. The UWP
Mica guidance page (/windows/uwp/ui-input/mica-uwp) is still live.

Co-authored-by: Copilot <[email protected]>

Author: GrantMeStrength

hub/apps/design/style/mica.md

@@ -130,6 +130,10 @@ To use Mica in a WinUI XAML app, see [Apply Mica or Acrylic materials in desktop
 
 To use Mica in a Win32 app, see [Apply Mica in Win32 desktop apps for Windows 11](../../desktop/modernize/ui/apply-mica-win32.md).
 
+### Use Mica with WinUI for UWP
+
+To use Mica in a UWP app with WinUI for UWP, see [Apply Mica with WinUI for UWP](/windows/uwp/ui-input/mica-uwp).
+
 ## Related articles
 
 - [Materials](../signature-experiences/materials.md)