Add dynamic DNS and show the running kernel on the overview

router.ddns keeps provider records on the uplink's addresses with ddclient,
run by a timer and by a watcher that starts it when an address changes. The
Dynamic DNS page edits it; the secret is a private file on the router. The
overview shows the running kernel and says when the current system boots a
different one. PLAN.md records why the A/B image approach was not pursued.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AeFqV9dZrK6nipHkPfyM4F

scripts/test.sh

@@ -6,7 +6,7 @@
 #                              and test_script() work; boot once, retry steps
 #   router-test --list
 # Logs: ${TMPDIR:-/tmp}/router-test-logs/NAME.log
-vm_tests="webui basic pppoe vlans onestick rollback dns installer apply setup update image wireguard"
+vm_tests="webui basic pppoe vlans onestick rollback dns ddns installer apply setup update image wireguard"
 flake=${FLAKE:-.}
 logs=${TMPDIR:-/tmp}/router-test-logs
 mkdir -p "$logs"