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/src/components/Sidebar.tsx

@@ -8,6 +8,7 @@ import { useQueries, useQuery } from "@tanstack/react-query";
 import { useTranslations } from "next-intl";
 import { fetchCollections, fetchContentList, getApiKey, clearSession } from "@/lib/api";
 import { LocaleSwitcher } from "./LocaleSwitcher";
+import { EnvironmentSwitcher } from "./EnvironmentSwitcher";
 
 const navLinkClass =
   "inline-flex items-center gap-2 rounded-lg px-3 py-2.5 text-sm font-medium min-h-[44px] md:min-h-0 md:py-2";
@@ -101,6 +102,9 @@ export function Sidebar({ locale, mobileOpen = false, onClose }: SidebarProps) {
             </button>
           )}
         </div>
+        <div className="flex shrink-0 flex-col gap-2 pb-1">
+          <EnvironmentSwitcher />
+        </div>
         <div className="flex shrink-0 flex-col gap-1">
           <Link
             href="/"