Configurable bind addresses + display host in Server Settings

ServerConfig grows two fields:
- BindAddresses: list of IPs Kestrel binds to (empty = all interfaces,
  current behavior). Listening only on a subset is useful when the host
  has multiple NICs and the webhook should not be reachable on all of
  them.
- DisplayHost: the hostname/IP the GUI splices into the URL column and
  Copy URL button. Cosmetic; doesn't affect what the server accepts.

Server Settings dialog gains a "Network" section: a checkbox for "all
interfaces" plus per-NIC checkboxes auto-detected via NetworkInterface.
GetAllNetworkInterfaces, and an editable ComboBox for the display host
pre-populated with detected IPs and the machine name.

Listener restart fires on BindAddresses change but not on DisplayHost
change (cosmetic).

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

src/WebhookServer.Gui/App.xaml

@@ -8,5 +8,6 @@
         <conv:BoolToBrushConverter x:Key="ConnFill"/>
         <conv:StringEqualsConverter x:Key="StringEqualsConverter"/>
         <conv:HookUrlConverter x:Key="HookUrl"/>
+        <conv:InvertBoolConverter x:Key="InvertBool"/>
     </Application.Resources>
 </Application>