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

tests/default.nix

@@ -9,6 +9,7 @@
   onestick = import ./onestick.nix { inherit self pkgs; };
   rollback = import ./rollback.nix { inherit self pkgs; };
   dns = import ./dns.nix { inherit self pkgs; };
+  ddns = import ./ddns.nix { inherit self pkgs; };
   installer = import ./installer.nix { inherit self pkgs; };
   apply = import ./apply.nix { inherit self pkgs; };
   webui = import ./webui.nix { inherit self pkgs; };