feat(config): admin Configuration page (registry-driven)

New admin-only Configuration page that renders the system_config registry grouped
(Email/SMTP first), with per-field save/reset, write-only encrypted secret fields
(disabled with a hint when TOKEN_ENCRYPTION_KEY is unset), and a Send-test-email
button. New 'config' page + sidebar nav item + header title; api methods and
EN/HU/DE strings.

frontend/src/components/NavSidebar.tsx

@@ -14,6 +14,7 @@ import {
   LogOut,
   Settings,
   Shield,
+  SlidersHorizontal,
   Tv,
   UserPlus,
 } from "lucide-react";
@@ -138,7 +139,10 @@ export default function NavSidebar({
   ];
   const systemItems: NavItem[] =
     me.role === "admin"
-      ? [{ page: "scheduler", icon: Activity, label: t("header.account.scheduler") }]
+      ? [
+          { page: "scheduler", icon: Activity, label: t("header.account.scheduler") },
+          { page: "config", icon: SlidersHorizontal, label: t("header.account.config") },
+        ]
       : [];
 
   const rowBase =