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/Views/ServerSettings.xaml.cs

@@ -0,0 +1,33 @@
+using System.Windows;
+using System.Windows.Controls;
+using WebhookServer.Gui.ViewModels;
+
+namespace WebhookServer.Gui.Views;
+
+public partial class ServerSettings : Window
+{
+    public ServerSettings()
+    {
+        InitializeComponent();
+    }
+
+    private void OnSave(object sender, RoutedEventArgs e)
+    {
+        if (DataContext is ServerSettingsViewModel vm)
+            vm.SaveCommand.Execute(null);
+        DialogResult = true;
+        Close();
+    }
+
+    private void OnPfxPasswordChanged(object sender, RoutedEventArgs e)
+    {
+        if (DataContext is ServerSettingsViewModel vm && sender is PasswordBox box)
+            vm.PfxPasswordInput = box.Password;
+    }
+
+    private void OnModeChecked(object sender, RoutedEventArgs e)
+    {
+        if (DataContext is ServerSettingsViewModel vm && sender is RadioButton rb && rb.Tag is string tag)
+            vm.HttpsMode = tag;
+    }
+}