test-vm: Serve whib-frontend with nginx

The WHIB frontend module no longer runs a container or has a `port`
option. Forward host port 8081 to nginx in the VM instead.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

hosts/test-vm/whib-frontend.nix

@@ -13,20 +13,22 @@ in
     services.whib-frontend = {
       enable = true;
 
-      domain = "whib-frontend.local";
-      port = "8081";
+      # nginx serves the frontend; matches the Host header of http://localhost:8081
+      domain = "localhost";
       # backendHost = "https://api.whib.ppp.pm/";
       backendHost = "http://localhost:8080";
     };
 
+    networking.firewall.allowedTCPPorts = [ 80 ];
+
     virtualisation.vmVariant = {
       virtualisation = {
         forwardPorts = [
           {
-            # Service API
+            # Frontend served by nginx
             from = "host";
             host.port = 8081;
-            guest.port = 8081;
+            guest.port = 80;
           }
         ];
       };