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/i18n/locales/de/header.json

@@ -5,6 +5,7 @@
   "usageStats": "Nutzung & Statistik",
   "scheduler": "Planer",
   "configuration": "Konfiguration",
+  "users": "Benutzer",
   "scope": {
     "label": "Feed-Quelle",
     "my": "Meine",
@@ -20,6 +21,7 @@
     "stats": "Statistik",
     "scheduler": "Planer",
     "config": "Konfiguration",
+    "users": "Benutzer",
     "settings": "Einstellungen",
     "about": "Über",
     "signOut": "Abmelden",