Replace ReactiveUI with CommunityToolkit.Mvvm in samples

[NeverMorewd]

Summary

• replace ReactiveUI and its source generators with CommunityToolkit.Mvvm in shared sample view models
• remove ReactiveUI platform packages and startup initialization from Avalonia, WPF, and WinUI samples
• preserve existing command and property binding names through CommunityToolkit source generation

Validation

• built Sample.Wpf successfully
• built Sample.Avalonia.Desktop successfully
• built Sample.WinUI successfully
• built Avalonia and WPF InfinityNavigation samples successfully
• built Sample.FrontDialog successfully
• verified no ReactiveUI, RxApp, UseReactiveUI, or System.Reactive references remain in sample source and project files