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>

.env.example

@@ -15,9 +15,9 @@ RUSTYCMS_API_KEY=dein-geheimes-token
 # Format: key1:role1,key2:role2. Roles: read (GET only), read_write (content + assets), admin (+ schemas).
 # RUSTYCMS_API_KEYS=key1:read,key2:read_write,key3:admin
 
-# Optional: environments (e.g. production,staging). File store only. Content under content/ and content/<env>/.
-# API: ?_environment=staging. Default = first in list.
-# RUSTYCMS_ENVIRONMENTS=production,staging
+# Optional: environments = spaces (e.g. one per website). File store only. First = content/, others = content/<env>/.
+# API: ?_environment=justpm for the other space. Default = first in list.
+# RUSTYCMS_ENVIRONMENTS=windwiderstand,justpm
 
 # Optional: webhook URLs (comma-separated). POST with JSON { event, collection?, slug?, ... } on content/asset/schema changes.
 # RUSTYCMS_WEBHOOKS=https://example.com/hook
@@ -34,3 +34,13 @@ RUSTYCMS_API_KEY=dein-geheimes-token
 # Optional: Paths to types and content directories. Useful for keeping content in a separate repo.
 # RUSTYCMS_TYPES_DIR=./types
 # RUSTYCMS_CONTENT_DIR=./content
+
+# --- Admin UI (Next.js, when running admin-ui e.g. npm run dev in admin-ui/) ---
+# Login: open /admin/login. Required for login to succeed.
+# ADMIN_USERNAME=admin
+# ADMIN_PASSWORD=your-secret-password
+# Optional: session cookie encryption. Change in production.
+# SESSION_SECRET=min-32-chars-secret-for-iron-session!!
+# Optional: API base URL and key for the admin (if not set, login returns API key from RUSTYCMS_API_KEY).
+# NEXT_PUBLIC_RUSTYCMS_API_URL=http://127.0.0.1:3000
+# NEXT_PUBLIC_RUSTYCMS_API_KEY=dein-geheimes-token