Phase 3: app icon (multi-resolution ICO + master PNG)

scripts/generate-icons.ps1 renders the icon programmatically with
System.Drawing - rounded teal square (#0E7C66) with a stylized white
hook glyph - at 16/24/32/48/64/128/256 px and assembles a proper
multi-resolution Microsoft ICO. The PNG and ICO outputs land in
resources/. The script is the source of truth; re-run after editing
the design.

GUI csproj uses ApplicationIcon for the EXE icon and embeds the .ico
+ .png as Resources so MainWindow and AboutDialog can use them via
WPF's resource URI scheme.

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

src/WebhookServer.Gui/MainWindow.xaml

@@ -7,6 +7,7 @@
         xmlns:models="clr-namespace:WebhookServer.Core.Models;assembly=WebhookServer.Core"
         mc:Ignorable="d"
         Title="Webhook Server" Height="600" Width="1000"
+        Icon="/webhook-server.ico"
         d:DataContext="{d:DesignInstance Type=vm:MainViewModel}">
     <Window.InputBindings>
         <KeyBinding Key="N" Modifiers="Control" Command="{Binding AddEndpointCommand}"/>