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/21-van-br0.network

@@ -1,5 +1,5 @@
 # The LAN gateway IP now lives on the bridge, not on the AP wlan (which is just a
-# bridge member). ConfigureWithoutCarrier keeps 10.42.0.1 up even before any wired
+# bridge member). ConfigureWithoutCarrier keeps 10.42.1.1 up even before any wired
 # device is plugged, exactly as it was on the wlan-only setup.
 [Match]
 Name=br0
@@ -8,7 +8,7 @@ Name=br0
 RequiredForOnline=no
 
 [Network]
-Address=10.42.0.1/24
+Address=@GATEWAY_IP@/@GATEWAY_PREFIX@
 ConfigureWithoutCarrier=yes
 # Resolve .local (mDNS) for devices on the van's own LAN (ESPHome nodes,
 # etc.) — scoped to br0 only, not the WAN links, since mDNS is link-local