Add environments (spaces) support

- API client sends _environment param on all content/asset requests
- GET /api/environments endpoint to list configured environments
- EnvironmentSwitcher in Sidebar with i18n label "Space"
- clearSession() also clears environment from sessionStorage
- .env.example documents ADMIN_USERNAME/PASSWORD/SESSION_SECRET

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

admin-ui/messages/de.json

@@ -10,6 +10,9 @@
     "invalidCredentials": "Ungültige Zugangsdaten",
     "networkError": "Netzwerkfehler, bitte erneut versuchen"
   },
+  "EnvironmentSwitcher": {
+    "label": "Space"
+  },
   "Sidebar": {
     "dashboard": "Dashboard",
     "types": "Typen",