roles/syncthingserver: use syncthingGuiPort variable

roles/syncthingserver.nix

@@ -13,6 +13,7 @@ with lib;
 
 let
   cfg = config.syncthingserver;
+  syncthingGuiPort = 8384;
 in
 {
   options.syncthingserver = {
@@ -82,7 +83,7 @@ in
         };
 
         http.services.syncthing-gui = {
-          loadBalancer.servers = [ { url = "http://localhost:8384"; } ];
+          loadBalancer.servers = [ { url = "http://localhost:${toString syncthingGuiPort}"; } ];
         };
       };
     };