refactor(ui): restructure sidebar and admin navigation

- Sidebar: remove footer (user info, settings, locale switcher), remove admin section, make category headers static with per-category collapse toggle
- Header dropdown: replace inline admin links with single "Administrator" entry
- Admin section: dedicated layout with AdminSidebar (mirrors Settings pattern) at /admin/*
- Disable self-delete button in user management (backend already rejects with 403)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/web/src/messages/de.json

@@ -69,6 +69,13 @@
     },
     "settings": "Einstellungen",
     "logout": "Abmelden",
+    "admin": {
+      "label": "Administrator",
+      "users": "Benutzer",
+      "tenants": "Mandanten",
+      "modules": "Module",
+      "ldap": "LDAP"
+    },
     "role": {
       "SUPER_ADMIN": "Super-Admin",
       "ADMIN": "Admin",