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/firewall.nix

@@ -226,7 +226,9 @@ in
             } } accept # wireguard"
           }
           ${lib.optionalString (dhcpIfaces != [ ]) "iifname ${set dhcpIfaces} udp dport 67 accept"}
-          ${lib.optionalString (mgmtIfaces != [ ]) "iifname ${set mgmtIfaces} tcp dport 22 accept"}
+          ${lib.optionalString (mgmtIfaces != [ ]) "iifname ${set mgmtIfaces} tcp dport { ${
+              lib.concatMapStringsSep ", " toString ([ 22 ] ++ config.router.managementPorts)
+            } } accept"}
         }
 
         chain forward {