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/Converters/Converters.cs

@@ -0,0 +1,35 @@
+using System.Globalization;
+using System.Windows.Data;
+using System.Windows.Media;
+
+namespace WebhookServer.Gui.Converters;
+
+public sealed class NullToBoolConverter : IValueConverter
+{
+    public object Convert(object? value, Type targetType, object? parameter, CultureInfo culture)
+        => value is not null;
+
+    public object ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)
+        => throw new NotSupportedException();
+}
+
+public sealed class StringEqualsConverter : IValueConverter
+{
+    public object Convert(object? value, Type targetType, object? parameter, CultureInfo culture)
+        => string.Equals(value as string, parameter as string, StringComparison.OrdinalIgnoreCase);
+
+    public object? ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)
+        => (value is bool b && b) ? parameter : Binding.DoNothing;
+}
+
+public sealed class BoolToBrushConverter : IValueConverter
+{
+    public Brush TrueBrush { get; set; } = Brushes.SeaGreen;
+    public Brush FalseBrush { get; set; } = Brushes.IndianRed;
+
+    public object Convert(object? value, Type targetType, object? parameter, CultureInfo culture)
+        => (value is bool b && b) ? TrueBrush : FalseBrush;
+
+    public object ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)
+        => throw new NotSupportedException();
+}