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/lib/urlState.ts

@@ -88,6 +88,7 @@ export const PAGES = [
   "playlists",
   "settings",
   "scheduler",
+  "config",
   "notifications",
 ] as const;