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

modules/settings/default.nix

@@ -66,6 +66,8 @@ let
   webui = pkgs.callPackage ../../pkgs/router-webui/package.nix { };
 in
 {
+  imports = [ ./update.nix ];
+
   # Outside `router`: the settings file defines that whole tree.
   options.routerSettings = {
     file = lib.mkOption {
@@ -154,7 +156,11 @@ in
       description = "Router web UI";
       wantedBy = [ "multi-user.target" ];
       after = [ "network.target" ];
-      path = [ config.systemd.package ];
+      # iproute2: the dashboard reads the default route and the PPP peer.
+      path = [
+        config.systemd.package
+        pkgs.iproute2
+      ];
       serviceConfig = {
         ExecStart = "${lib.getExe webui} -listen :${toString cfg.webui.port} -config ${cfg.directory} -state ${cfg.stateDirectory}";
         Restart = "on-failure";