fix: open ports 80/443 in proxy module when proxies are configured

The greg.proxies module enables nginx but never opened the firewall,
so all proxied services (Jellyfin, Grafana, etc.) were unreachable
on the LAN. Add allowedTCPPorts [80 443] conditioned on the same
mkIf guard as the nginx virtualHosts config.

modules/nixos/proxy.nix

@@ -92,4 +92,9 @@ with lib;
 
     virtualHosts = mapAttrs makeHost cfg;
   };
+
+  config.networking.firewall.allowedTCPPorts = mkIf ((attrValues cfg) != [ ]) [
+    80
+    443
+  ];
 }