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

pkgs/router-webui/templates/dashboard.html

@@ -33,6 +33,7 @@
     <h2>System</h2>
     <dl>
       <dt>Uptime</dt><dd>{{age $sys.Uptime}}</dd>
+      {{if $sys.Kernel}}<dt>Kernel</dt><dd>{{$sys.Kernel}}{{if $sys.KernelNext}} <span class="bad">reboot to run {{$sys.KernelNext}}</span>{{end}}</dd>{{end}}
       <dt>Load</dt><dd>{{$sys.Load}}</dd>
       <dt>CPU</dt><dd>{{value $sys.CPU "%"}}</dd>
       {{if $sys.MemTotal}}<dt>Memory</dt><dd>{{bytes $sys.MemUsed}} of {{bytes $sys.MemTotal}}</dd>{{end}}