Make router IP address configurable via deploy.conf

Add GATEWAY_IP, GATEWAY_PREFIX, GATEWAY_NETWORK, DHCP_RANGE_START, and
DHCP_RANGE_END variables to deploy.conf. Update ap/21-van-br0.network,
ap/van-ap-dnsmasq.conf, and ap/nftables.conf to use @TOKEN@ placeholders
for IP configuration instead of hardcoding 10.42.x addresses.

This allows deploying the same configuration to multiple routers on
different networks (e.g., Pi 5 on 10.42.0.x and R5C on 10.42.1.x) by
simply editing deploy.conf before running ./deploy.sh.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

ap/nftables.conf

@@ -6,6 +6,6 @@ delete table ip van_router_nat
 table ip van_router_nat {
     chain postrouting {
         type nat hook postrouting priority srcnat; policy accept;
-        ip saddr 10.42.0.0/24 oifname != "br0" masquerade
+        ip saddr @GATEWAY_NETWORK@ oifname != "br0" masquerade
     }
 }