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

@@ -0,0 +1,87 @@
+<Window x:Class="WebhookServer.Gui.MainWindow"
+        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+        xmlns:vm="clr-namespace:WebhookServer.Gui.ViewModels"
+        xmlns:models="clr-namespace:WebhookServer.Core.Models;assembly=WebhookServer.Core"
+        mc:Ignorable="d"
+        Title="Webhook Server" Height="600" Width="1000"
+        d:DataContext="{d:DesignInstance Type=vm:MainViewModel}">
+    <DockPanel LastChildFill="True">
+        <StatusBar DockPanel.Dock="Bottom">
+            <StatusBarItem>
+                <Ellipse Width="10" Height="10"
+                         Fill="{Binding IsConnected, Converter={StaticResource ConnFill}}"/>
+            </StatusBarItem>
+            <StatusBarItem>
+                <TextBlock Text="{Binding ConnectionStatus}"/>
+            </StatusBarItem>
+        </StatusBar>
+
+        <ToolBar DockPanel.Dock="Top">
+            <Button Content="Refresh" Command="{Binding RefreshCommand}"/>
+            <Separator/>
+            <Button Content="Add" Command="{Binding AddEndpointCommand}"/>
+            <Button Content="Edit" Command="{Binding EditEndpointCommand}"
+                    IsEnabled="{Binding SelectedEndpoint, Converter={StaticResource NotNull}}"/>
+            <Button Content="Delete" Command="{Binding DeleteEndpointCommand}"
+                    IsEnabled="{Binding SelectedEndpoint, Converter={StaticResource NotNull}}"/>
+            <Separator/>
+            <Button Content="Server Settings…" Command="{Binding EditServerSettingsCommand}"/>
+        </ToolBar>
+
+        <Grid>
+            <Grid.RowDefinitions>
+                <RowDefinition Height="*"/>
+                <RowDefinition Height="5"/>
+                <RowDefinition Height="200"/>
+            </Grid.RowDefinitions>
+
+            <DataGrid Grid.Row="0"
+                      ItemsSource="{Binding Endpoints}"
+                      SelectedItem="{Binding SelectedEndpoint, Mode=TwoWay}"
+                      AutoGenerateColumns="False"
+                      CanUserAddRows="False"
+                      CanUserDeleteRows="False"
+                      IsReadOnly="True"
+                      HeadersVisibility="Column">
+                <DataGrid.Columns>
+                    <DataGridTemplateColumn Header="Enabled" Width="80">
+                        <DataGridTemplateColumn.CellTemplate>
+                            <DataTemplate DataType="{x:Type models:EndpointConfig}">
+                                <CheckBox IsChecked="{Binding Enabled, Mode=OneWay}"
+                                          Command="{Binding DataContext.ToggleEnabledCommand, RelativeSource={RelativeSource AncestorType=Window}}"
+                                          CommandParameter="{Binding}"/>
+                            </DataTemplate>
+                        </DataGridTemplateColumn.CellTemplate>
+                    </DataGridTemplateColumn>
+                    <DataGridTextColumn Header="Slug" Binding="{Binding Slug}" Width="*"/>
+                    <DataGridTextColumn Header="Auth" Binding="{Binding AuthMode}" Width="80"/>
+                    <DataGridTextColumn Header="Executor" Binding="{Binding ExecutorType}" Width="140"/>
+                    <DataGridTextColumn Header="Mode" Binding="{Binding ResponseMode}" Width="80"/>
+                    <DataGridTextColumn Header="Description" Binding="{Binding Description}" Width="2*"/>
+                </DataGrid.Columns>
+            </DataGrid>
+
+            <GridSplitter Grid.Row="1" HorizontalAlignment="Stretch" Background="#DDD"/>
+
+            <DockPanel Grid.Row="2">
+                <Grid DockPanel.Dock="Top">
+                    <Grid.ColumnDefinitions>
+                        <ColumnDefinition Width="*"/>
+                        <ColumnDefinition Width="Auto"/>
+                    </Grid.ColumnDefinitions>
+                    <TextBlock Text="Recent log entries" FontWeight="Bold" Margin="6,4"/>
+                    <Button Grid.Column="1" Content="Refresh" Command="{Binding RefreshLogTailCommand}" Margin="6,2"/>
+                </Grid>
+                <TextBox Text="{Binding LogTail, Mode=OneWay}"
+                         IsReadOnly="True"
+                         FontFamily="Consolas"
+                         VerticalScrollBarVisibility="Auto"
+                         HorizontalScrollBarVisibility="Auto"
+                         TextWrapping="NoWrap"/>
+            </DockPanel>
+        </Grid>
+    </DockPanel>
+</Window>