Phase 5: tray icon with minimize-to-tray and context menu

GUI csproj enables UseWindowsForms (NotifyIcon lives in WinForms even
in .NET 8). New Services/TrayIcon.cs wraps NotifyIcon with a context
menu (Open / Restart service / Exit) and the embedded webhook-server
icon. MainWindow creates the TrayIcon, hides itself on minimize and
restores on tray double-click.

Adds GlobalUsings.cs to alias the WPF defaults for types that exist
in both WPF and WinForms (Application, MessageBox, TextBox, Binding,
etc.) so existing code keeps compiling without per-file changes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/WebhookServer.Gui/MainWindow.xaml.cs

@@ -8,12 +8,37 @@ namespace WebhookServer.Gui;
 
 public partial class MainWindow : Window
 {
+    private readonly TrayIcon _tray;
+    private readonly MainViewModel _vm;
+
     public MainWindow()
     {
         InitializeComponent();
-        var vm = new MainViewModel(new AdminPipeClient());
-        DataContext = vm;
-        Loaded += async (_, _) => await vm.RefreshCommand.ExecuteAsync(null);
+        _vm = new MainViewModel(new AdminPipeClient());
+        DataContext = _vm;
+
+        _tray = new TrayIcon(
+            resolveMainWindow: () => Application.Current.MainWindow,
+            restartServiceAsync: async () => await new AdminPipeClient().RestartListenerAsync());
+
+        Loaded += async (_, _) => await _vm.RefreshCommand.ExecuteAsync(null);
+        StateChanged += OnStateChanged;
+        Closed += (_, _) => _tray.Dispose();
+    }
+
+    private void OnStateChanged(object? sender, EventArgs e)
+    {
+        // Minimize-to-tray: hide the window when the user minimizes; restoring is
+        // via the tray icon's double-click or context menu.
+        if (WindowState == WindowState.Minimized)
+        {
+            Hide();
+            ShowInTaskbar = false;
+        }
+        else
+        {
+            ShowInTaskbar = true;
+        }
     }
 
     private void OnLogTailChanged(object sender, TextChangedEventArgs e)