feat(auth): admin Users page + allow_registration toggle (5a frontend)

New admin-only Users page (sidebar): Users & roles (list + promote/demote with
confirm; self/demo/last-admin guarded server-side), plus the Access requests
(Invite whitelist) and Demo whitelist+reset migrated out of Settings → Account
(same data/tables — UI relocated only). Settings → Account now holds personal
content only. ConfigPanel learns a boolean field type (toggle) for the new
allow_registration setting. api methods, new 'users' page/route/nav/header, and
EN/HU/DE strings (new users namespace + access group).

frontend/src/components/NavSidebar.tsx

@@ -15,6 +15,7 @@ import {
   Settings,
   Shield,
   SlidersHorizontal,
+  Users,
   Tv,
   UserPlus,
 } from "lucide-react";
@@ -142,6 +143,7 @@ export default function NavSidebar({
       ? [
           { page: "scheduler", icon: Activity, label: t("header.account.scheduler") },
           { page: "config", icon: SlidersHorizontal, label: t("header.account.config") },
+          { page: "users", icon: Users, label: t("header.account.users") },
         ]
       : [];