[ADD] SystemBackdropElement Control Sample Page

[rashmi-thakurr]

Description

Adds Samples for SystemBackdropElement Control

Motivation and Context

Provides a clearer example of how to use SystemBackdropElement. Helps developers understand and test the newer backdrop APIs.

How Has This Been Tested?

Tested in WinUI Gallery on Windows 11. Verified backdrop switching, CornerRadius updates, and sample loading without errors in Debug and Release.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• [x ] New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

[rashmi-thakurr]

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[niels9001]

@rashmi-thakurr Exciting to see this new feature land :)!

The main branch is used for the latest stable release. Therefore, this PR should merge into: the 2.0 branch. Once 2.0 hits stable we will then merge that branch into main

[niels9001]

LGTM! Great new sample for a great new API 😁