feat: add Environment admin UI page

SplitPane with create/edit/delete, production flag toggle,
enabled/disabled toggle. Follows existing admin page patterns.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/components/sidebar-utils.ts

@@ -98,6 +98,7 @@ export function buildAppTreeNodes(
  */
 export function buildAdminTreeNodes(): SidebarTreeNode[] {
   return [
+    { id: 'admin:environments', label: 'Environments', path: '/admin/environments' },
     { id: 'admin:rbac', label: 'Users & Roles', path: '/admin/rbac' },
     { id: 'admin:audit', label: 'Audit Log', path: '/admin/audit' },
     { id: 'admin:oidc', label: 'OIDC', path: '/admin/oidc' },