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/App.xaml.cs

@@ -1,13 +1,5 @@
-using System.Configuration;
-using System.Data;
-using System.Windows;
-
 namespace WebhookServer.Gui;
 
-/// <summary>
-/// Interaction logic for App.xaml
-/// </summary>
 public partial class App : Application
 {
 }
-