Template remaining host-specific values (WAN wifi, LAN/eth0, NVMe, nftables)

nftables.conf hardcoded the R5C network, van-br0-setup.service the R5C
interfaces and several configs wlan0/eth0. All now come from the host
config; the onboard LAN port is skipped when ONBOARD_LAN_IFACE is empty.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

failover/config.json

@@ -7,7 +7,7 @@
     "http://connectivity-check.ubuntu.com/"
   ],
   "wans": [
-    { "name": "wifi",     "device": "wlan0",           "metric": 100 },
+    { "name": "wifi",     "device": "@WIFI_WAN_IFACE@",           "metric": 100 },
     { "name": "starlink", "device": "@STARLINK_IFACE@", "metric": 200 },
     { "name": "cellular", "connection": "Koodo",       "metric": 300 }
   ]