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.Core/Ipc/AdminProtocol.cs

@@ -52,6 +52,7 @@ public sealed class StatusInfo
     public bool Running { get; set; }
     public int HttpPort { get; set; }
     public int? HttpsPort { get; set; }
+    public string? DisplayHost { get; set; }
     public DateTimeOffset StartedAt { get; set; }
     public int EndpointCount { get; set; }
 }