Improve performance when switching categories in the MAUI sample viewer

[duffh]

Description

Rewrote the Category page in the MAUI viewer to use CollectionView rather than a nested FlexLayout inside a ScrollView. This was killing performance.

Type of change

• Sample viewer enhancement

Platforms tested on

• WPF .NET 8
• WPF Framework
• WinUI
• MAUI WinUI
• MAUI Android
• MAUI iOS
• MAUI MacCatalyst

Checklist

• Self-review of changes
• All changes work as expected on all affected platforms
• There are no warnings related to changes
• Code is commented and follows .NET conventions and standards
• Codemaid and XAML styler extensions have been run on every changed file
• No unrelated changes have been made to any other code or project files