Add a dashboard and backup download to the web UI

Metric history (5s to 1h tiers, persisted), uplink status, ping targets,
DHCP leases, and a tar.gz backup of the state directory.

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

pkgs/router-webui/templates/advanced.html

@@ -1,9 +1,31 @@
 {{define "content"}}
+<section>
+<h2>Settings</h2>
 <p>The complete settings: the <code>router</code> options of nixos-router as JSON, plus <code>system</code>.
    They are checked when applied; nothing changes until then.</p>
 <form method="post" action="/advanced">
   <input type="hidden" name="csrf" value="{{.CSRF}}">
-  <textarea name="settings" rows="30" spellcheck="false">{{.Data}}</textarea>
+  <textarea name="settings" rows="30" spellcheck="false">{{.Data.Settings}}</textarea>
   <button>Save</button>
 </form>
+</section>
+
+<section>
+<h2>Dashboard</h2>
+<form method="post" action="/prefs" class="narrow">
+  <input type="hidden" name="csrf" value="{{.CSRF}}">
+  <label>Ping targets <input name="pingTargets" value="{{.Data.PingTargets}}" spellcheck="false"></label>
+  <p class="hint">Up to 5 IP addresses, separated by commas. <code>gateway</code> is the provider's end of the line.
+     Takes effect right away; no apply needed.</p>
+  <button>Save</button>
+</form>
+</section>
+
+<section>
+<h2>Backup</h2>
+<p>Everything on <code>/var/lib/router</code>: settings, PPPoE password, WireGuard keys, passwords, the web UI's
+   certificate and the dashboard history. Keep the file safe. To restore, put it on a USB stick and run
+   <code>router-setup</code> from the installer: it offers to install from it.</p>
+<form method="post" action="/backup"><input type="hidden" name="csrf" value="{{.CSRF}}"><button>Download backup</button></form>
+</section>
 {{end}}