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/App.xaml

@@ -0,0 +1,11 @@
+<Application x:Class="WebhookServer.Gui.App"
+             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:conv="clr-namespace:WebhookServer.Gui.Converters"
+             StartupUri="MainWindow.xaml">
+    <Application.Resources>
+        <conv:NullToBoolConverter x:Key="NotNull"/>
+        <conv:BoolToBrushConverter x:Key="ConnFill"/>
+        <conv:StringEqualsConverter x:Key="StringEqualsConverter"/>
+    </Application.Resources>
+</Application>