Add a web UI for settings-configured routers

router-webui (Go, standard library only) edits settings.json: host
name, WAN (DHCP or PPPoE), LANs with DHCP, and the raw JSON for the
rest; keys the forms do not know are kept. It applies and confirms by
starting router-apply.service and router-confirm.service, which a
switch never restarts, and shows the build log and the rollback
countdown. HTTPS with a self-signed certificate, a PBKDF2 password
hash, CSRF tokens and an Origin check; reachable from management LANs
only (new router.managementPorts).

router-apply records the settings it built and router-confirm keeps
them as settings.confirmed.json (new rollback.onConfirm hook), so the
UI can tell unapplied changes and discard them.

checks.apply now also logs in from a LAN client, discards the change
left by the rollback, edits the DHCP pool, applies and confirms.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

modules/router/options.nix

@@ -481,6 +481,16 @@ in
       );
     };
 
+    managementPorts = mkOption {
+      type = types.listOf types.port;
+      default = [ ];
+      example = [ 443 ];
+      description = ''
+        TCP ports on the router, besides SSH, reachable from LANs and tunnels
+        with `allowManagement` (the web UI adds its own).
+      '';
+    };
+
     dns.domain = mkOption {
       type = types.nullOr types.str;
       default = null;