Add default case with exception to navigation switch statement

[Copilot]

The NavView_SelectionChanged handler in the NavigationApp C# template had no fallback for unrecognized NavigationViewItem tags, silently swallowing unknown navigation items.

Changes

• MainWindow.xaml.cs: Added default case to the switch (item.Tag) block that throws InvalidOperationException with the unknown tag value, making misconfigurations immediately visible at runtime.

switch (item.Tag)
{
    case "home":
        NavFrame.Navigate(typeof(HomePage));
        break;
    case "about":
        NavFrame.Navigate(typeof(AboutPage));
        break;
    default:
        throw new InvalidOperationException($"Unknown navigation item tag: {item.Tag}");
}

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