Fix RadioButton keyboard navigation in ItemsRepeater layout options

[Copilot]

This PR fixes an accessibility issue where the "Uniform grid" and "Custom virtualizing layout" radio buttons in the ItemsRepeater sample page had incorrect keyboard navigation behavior.

Problem

The radio buttons were implemented as individual RadioButton elements with Click handlers, which caused both Tab and Arrow key navigation to work within the same group. This violated accessibility guidelines and confused keyboard users who expected:

• Tab key to move focus into/out of the radio group
• Arrow keys to navigate within the group and auto-select items
• No Space key requirement for selection

Solution

Replaced the individual RadioButton elements with a RadioButtons control:

Before:

<RadioButton Click="LayoutBtn_Click" Content="Uniform grid" GroupName="Sample2Layouts" Tag="UniformGridLayout2" />
<RadioButton Click="LayoutBtn_Click" Content="Custom virtualizing layout" GroupName="Sample2Layouts" IsChecked="True" Tag="MyFeedLayout" />

After:

<RadioButtons SelectedIndex="1" SelectionChanged="LayoutBtn_SelectionChanged">
    <RadioButton Content="Uniform grid" Tag="UniformGridLayout2" />
    <RadioButton Content="Custom virtualizing layout" Tag="MyFeedLayout" />
</RadioButtons>

Updated the corresponding C# event handler to use SelectionChangedEventArgs and get the selected item from the RadioButtons control while maintaining all existing layout switching functionality.

Benefits

• ✅ Proper keyboard navigation (Tab moves between groups, Arrow keys within groups)
• ✅ Auto-selection with arrow keys (no Space key required)
• ✅ Follows WinUI accessibility guidelines
• ✅ Consistent with other RadioButtons usage in the Gallery
• ✅ Improved experience for keyboard-only users

Fixes #1943.

Warning

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[niels9001]

LGTM! Tested, and it works! Amazing!

[marcelwgn]

/azp run

[marcelwgn]

/azp run