feat: add native nftables support (#1389)

- Add nftables as a third firewall backend option alongside firewalld
and iptables
- Detection priority: firewalld → nftables → iptables (legacy fallback)
- Uses dedicated `openvpn` and `openvpn-nat` tables for clean isolation
- Integrates with native `nftables.service` via include in
`/etc/nftables.conf`


Closes https://github.com/angristan/openvpn-install/issues/530

docker-compose.yml

@@ -6,6 +6,8 @@ services:
       dockerfile: test/Dockerfile.server
       args:
         BASE_IMAGE: ${BASE_IMAGE:-ubuntu:24.04}
+        ENABLE_FIREWALLD: ${ENABLE_FIREWALLD:-n}
+        ENABLE_NFTABLES: ${ENABLE_NFTABLES:-n}
     container_name: openvpn-server
     hostname: openvpn-server
     privileged: true