How do you intercept the hardware/system back button?

Override OnBackButtonPressed on a Page (Android hardware back) and return true to indicate you've handled it (prevent default). For Shell navigation interception, use the Navigating event. iOS has no hardware back, so also handle the navigation-bar back through Shell or a custom toolbar item.

protected override bool OnBackButtonPressed()
{
    if (HasUnsavedChanges) { PromptDiscard(); return true; }
    return base.OnBackButtonPressed();
}