Add support for nested submenus in TrayIconHost

[Copilot]

TrayMenu.Open() only built a flat menu structure, ignoring the Menu property on TrayMenuItem. This prevented creating multi-level context menus for the system tray icon.

Changes

• User32.cs: Added MF_POPUP flag and AppendMenu overload accepting nint for submenu handles
• TrayIconHost.cs: Extracted menu building into recursive BuildMenu helper that creates submenus when TrayMenuItem.Menu is populated

Usage

Menu = new TrayMenu
{
    new TrayMenuItem
    {
        Header = "Settings",
        Menu = new TrayMenu
        {
            new TrayMenuItem { Header = "Option 1", Command = Opt1Command },
            new TrayMenuItem
            {
                Header = "Advanced",
                Menu = new TrayMenu
                {
                    new TrayMenuItem { Header = "Sub-option A" },
                    new TrayMenuItem { Header = "Sub-option B" },
                }
            }
        }
    },
    new TrayMenuItem { Header = "Exit", Command = ExitCommand }
}

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Inability to Create Nested Submenus using TrayIconHost</issue_title>
<issue_description>### Describe the bug

Attempting to implement nested submenus (submenus within an existing submenu) for the system tray icon context menu fails; only the first level of submenus can be successfully displayed. Is this a known limitation of the current implementation, or is there a specific method/pattern required to successfully implement nested submenus that I might be missing?

To Reproduce

using the example code:

using Win32Icon icon = new(ResourcesProvider.GetStream(new Uri("pack://application:,,,/Wpf.Ui.Test;component/Resources/Images/ProfilePicture.ico")));

 _iconHost = new TrayIconHost()
 {
     ToolTipText = "Wpf.Ui.Test",
     Icon = icon.Handle,
     Menu =
     [
         new TrayMenuItem()
         {
             Header = Version,
             IsEnabled = false,
         },
         new TraySeparator(),
         new TrayMenuItem()
         {
             Header = "Restart",
             Command = RestartCommand,
         },
         new TrayMenuItem()
         {
             Header = "Exit",
             Command = ExitCommand,
         }
     ],
 };

I can't create nested submenus

Expected behavior

The nested submenu (Level 2) should appear when the user hovers over the Level 1 submenu item in the system tray context menu, allowing for a structured menu hierarchy.

Screenshots

No response

OS version

Windows 10 19045.5011

.NET version

8

WPF-UI NuGet version

4.0.3.6

Additional context

No response</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Inability to Create Nested Submenus using TrayIconHost #32

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