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/i18n/locales/de/header.json

@@ -4,6 +4,7 @@
   "channelManager": "Kanalverwaltung",
   "usageStats": "Nutzung & Statistik",
   "scheduler": "Planer",
+  "configuration": "Konfiguration",
   "scope": {
     "label": "Feed-Quelle",
     "my": "Meine",
@@ -18,6 +19,7 @@
     "playlists": "Wiedergabelisten",
     "stats": "Statistik",
     "scheduler": "Planer",
+    "config": "Konfiguration",
     "settings": "Einstellungen",
     "about": "Über",
     "signOut": "Abmelden",