public async void Enter_Button(object sender, EventArgs e)
{
var page = new FirstPage();
page.BackgroundColor = Color.Moccasin;
await Navigation.PushAsync(page, true);
}
// обработка нажатия элемента в списке
private async void OnItemSelected(object sender, SelectedItemChangedEventArgs e)
{
Friend selectedFriend = (Friend)e.SelectedItem;
FirstPage friendPage = new FirstPage();
friendPage.BindingContext = selectedFriend;
await Navigation.PushAsync(friendPage);
}