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>

power/nvme-watch-config.json

@@ -1,6 +1,6 @@
 {
-  "nvme_device": "nvme0",
-  "fs_device": "/dev/nvme0n1p2",
+  "nvme_device": "@NVME_DEVICE@",
+  "fs_device": "@NVME_FS_DEVICE@",
   "grace_period": 20,
   "cooldown": 300,
   "credentials_path": "/etc/van-battery/pushover.json"