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/ddns.html

@@ -0,0 +1,50 @@
+{{define "content"}}
+{{with .Data}}
+{{$e := .Errors}}{{$f := .Form}}
+<p class="hint">Keeps DNS records at your provider pointing at the router's internet addresses: it checks regularly and
+   right away when the address changes (a new PPPoE session, a new lease). Uses
+   <a href="https://ddclient.net/protocols.html">ddclient</a>; one provider account.</p>
+<form method="post" action="/ddns">
+  <input type="hidden" name="csrf" value="{{$.CSRF}}">
+  <fieldset>
+    <legend>Provider</legend>
+    <label class="inline"><input type="checkbox" name="enabled" {{if $f.Enabled}}checked{{end}}> Keep these names up to date</label>
+    <div>
+    <label>Provider (protocol) <input name="protocol" value="{{$f.Protocol}}" size="16" list="protocols" spellcheck="false"></label>
+    <datalist id="protocols"><option value="cloudflare"><option value="duckdns"><option value="dyndns2"><option value="namecheap"><option value="porkbun"><option value="hetzner"><option value="godaddy"><option value="digitalocean"></datalist>
+    <label>Server <input name="server" value="{{$f.Server}}" size="26" spellcheck="false" placeholder="only for dyndns2"></label>
+    <label>Zone <input name="zone" value="{{$f.Zone}}" size="22" spellcheck="false" placeholder="example.org (Cloudflare)"></label>
+    </div>
+    {{range $k := list "protocol" "server" "zone"}}{{with index $e $k}}<p class="field-error">{{$k}}: {{.}}</p>{{end}}{{end}}
+    <div>
+    <label>Login <input name="username" value="{{$f.Username}}" size="18" spellcheck="false" autocomplete="off"></label>
+    <label>Password or API token <input type="password" name="secret" size="30" autocomplete="new-password"
+      placeholder="{{if $f.SecretSet}}stored; leave empty to keep{{else}}required{{end}}"></label>
+    </div>
+    {{with index $e "secret"}}<p class="field-error">{{.}}</p>{{end}}
+    <p class="hint">Login is <code>token</code> for a Cloudflare API token (permission Zone → DNS → Edit). The secret is kept on the router
+       in a private file, not in the settings.</p>
+  </fieldset>
+  <fieldset>
+    <legend>Names</legend>
+    <label>Host names, one per line <textarea name="domains" rows="6" cols="40" spellcheck="false">{{$f.Domains}}</textarea></label>
+    {{with index $e "domains"}}<p class="field-error">{{.}}</p>{{end}}
+    <div>
+    <label class="inline"><input type="checkbox" name="ipv4" {{if $f.IPv4}}checked{{end}}> IPv4 (A records)</label>
+    <label class="inline"><input type="checkbox" name="ipv6" {{if $f.IPv6}}checked{{end}}> IPv6 (AAAA records)</label>
+    <label class="inline"><input type="checkbox" name="ssl" {{if $f.SSL}}checked{{end}}> HTTPS to the provider</label>
+    <label>Check every <input name="interval" value="{{$f.Interval}}" size="6"></label>
+    </div>
+    {{range $k := list "ipv4" "interval"}}{{with index $e $k}}<p class="field-error">{{$k}}: {{.}}</p>{{end}}{{end}}
+  </fieldset>
+  <button>Save</button>
+</form>
+
+<section>
+  <h2>Last updates</h2>
+  {{if .Status.Log}}<p class="muted">ddclient: {{or .Status.State "unknown"}}{{if .Status.Last}}, last run {{.Status.Last}}{{end}}</p>
+  <pre class="log">{{.Status.Log}}</pre>
+  {{else}}<p class="muted">Nothing yet. Once applied, the log of ddclient shows up here.</p>{{end}}
+</section>
+{{end}}
+{{end}}