Initial WebhookServer implementation

Add the .NET 8 solution scaffolded against PLAN.md. Three projects share
WebhookServer.Core (models, auth, execution, storage, IPC, callbacks)
and WebhookServer.Service hosts an embedded Kestrel listener plus the
named-pipe admin server. WebhookServer.Gui is a thin MVVM client over
the pipe. Includes 25 unit tests covering HMAC verification, bearer
auth, IP allowlist parsing, arg-template rendering, DPAPI round-trip,
and the encrypt-on-save config store.

Install/uninstall PowerShell scripts default to LocalSystem and accept
a domain user or gMSA via -ServiceAccount.

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

src/WebhookServer.Gui/MainWindow.xaml.cs

@@ -0,0 +1,16 @@
+using System.Windows;
+using WebhookServer.Gui.Services;
+using WebhookServer.Gui.ViewModels;
+
+namespace WebhookServer.Gui;
+
+public partial class MainWindow : Window
+{
+    public MainWindow()
+    {
+        InitializeComponent();
+        var vm = new MainViewModel(new AdminPipeClient());
+        DataContext = vm;
+        Loaded += async (_, _) => await vm.RefreshCommand.ExecuteAsync(null);
+    }
+}