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/Header.tsx

@@ -75,7 +75,9 @@ export default function Header({
                 ? t("settings.title")
                 : page === "scheduler"
                   ? t("header.scheduler")
-                  : t("header.channelManager")}
+                  : page === "config"
+                    ? t("header.configuration")
+                    : t("header.channelManager")}
         </div>
       )}
     </header>