Fix context menu background not following dark theme

[Copilot]

WPF ContextMenu popups open via a PopupRoot with InheritanceBehavior.SkipAllNext, which severs propagation of iNKORE.UI.WPF.Modern's inheritable ThemeProperty. As a result, all context menus rendered light regardless of the application theme.

Changes

• App.xaml.cs: Register a class-level handler for ContextMenu.OpenedEvent that explicitly calls ThemeManager.SetRequestedTheme with the current application theme on every context menu open. This injects the correct theme resources directly into the context menu's ResourceDictionary, covering both gear-button and right-click menus, and handles runtime theme switches.

EventManager.RegisterClassHandler(typeof(ContextMenu), ContextMenu.OpenedEvent,
    new RoutedEventHandler(OnContextMenuOpened));

private static void OnContextMenuOpened(object sender, RoutedEventArgs e)
{
    if (sender is ContextMenu contextMenu)
    {
        var elementTheme = ThemeManager.Current.ActualApplicationTheme == ApplicationTheme.Dark
            ? ElementTheme.Dark : ElementTheme.Light;
        ThemeManager.SetRequestedTheme(contextMenu, elementTheme);
    }
}