router-status: replace Cockpit's Van Router plugin with MQTT -> Home Assistant

Cockpit's login was unreliable (Firefox rejected sometimes, Android failed
outright with a TLS handshake error) and it was a resource hog on this Pi.
van-router-mqtt publishes the same AP/client/WAN/Starlink/modem status and
accepts the same control actions (AP/modem restart, WAN connect/disconnect,
prefer) via MQTT with HA discovery, using the local Mosquitto broker already
running here. It republishes van-failover/van-thermal's own state.json files
verbatim rather than modifying either daemon (van-thermal is deliberately
stdlib-only; van-failover's control loop wasn't worth an added dependency).

Adds a Lovelace dashboard (ha/dashboards/van-router.yaml) consuming those
entities. Cockpit's plugin is left deployed for now, not yet retired.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S2tpeaiHUriQbbnQ4SCbkf

router-status/van-router-mqtt.service

@@ -0,0 +1,13 @@
+[Unit]
+Description=Van Router status/control -> MQTT (Home Assistant discovery)
+After=NetworkManager.service network-online.target mosquitto.service
+Wants=NetworkManager.service
+
+[Service]
+Type=simple
+ExecStart=/usr/local/sbin/van-router-mqtt
+Restart=always
+RestartSec=5
+
+[Install]
+WantedBy=multi-user.target