private UserControl? _activeView;

internal void NavigateTo(UserControl view)
{
    try
    {
        if (_activeView != null)
        {
            MainContent.Children.Remove(_activeView);
        }

        _activeView = view;

        // Настраиваем поведение "Fill"
        view.HorizontalAlignment = HorizontalAlignment.Stretch;
        view.VerticalAlignment = VerticalAlignment.Stretch;
        view.Width = double.NaN;  // Аналог Auto в XAML
        view.Height = double.NaN; // Аналог Auto в XAML

        MainContent.Children.Add(view);
    }
    catch (Exception ex) { MessageBox.Show($"Ошибка навигации: {ex.Message}", "Ошибка", MessageBoxButton.OK, MessageBoxImage.Error); }
}