feat: add RBAC management UI with dashboard, users, groups, and roles tabs

Tab-based admin page at /admin/rbac with split-pane entity views,
inheritance visualization, OIDC badges, and role/group management.

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

ui/src/components/layout/AppSidebar.tsx

@@ -124,6 +124,7 @@ const ADMIN_LINKS = [
   { to: '/admin/opensearch', label: 'OpenSearch' },
   { to: '/admin/audit', label: 'Audit Log' },
   { to: '/admin/oidc', label: 'OIDC' },
+  { to: '/admin/rbac', label: 'User Management' },
 ];
 
 function AdminSubMenu({ collapsed: sidebarCollapsed }: { collapsed: boolean }) {