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/Converters/Converters.cs

@@ -1,6 +1,7 @@
 using System.Globalization;
 using System.Windows.Data;
-using System.Windows.Media;
+using Brush = System.Windows.Media.Brush;
+using Brushes = System.Windows.Media.Brushes;
 
 namespace WebhookServer.Gui.Converters;