refactor: full-stack restructure with multi-tenancy, workspace management, and K8s diagnostics

- Add Workspace domain (entity, repository, service, handler, DTO)
- Add multi-tenant K8s client with tenant binding and quota management
- Add K8s diagnostics client (instance diagnostics)
- Add authorization middleware (authz package)
- Restructure frontend to feature-based architecture (features/)
- Add User Management page in configuration
- Add AccessDenied page and route guards
- Refactor shared components (form inputs, layout, UI)
- Update Tailwind config for new design system
- Add comprehensive documentation (docs/, tasks/, plans)
- Improve cluster service with better kubeconfig handling
- Add tests for crypto, config, helm client, tenant binding

frontend/src/shared/components/feedback/EmptyStateSimple.tsx

@@ -23,10 +23,10 @@ export const EmptyStateSimple: React.FC<EmptyStateSimpleProps> = ({
   className = "",
 }) => {
   return (
-    <div className={`bg-gray-800/30 border border-gray-700 rounded-lg p-8 text-center ${className}`}>
+    <div className={`bg-white border border-slate-200 rounded-lg p-8 text-center ${className}`}>
       <Icon className="w-16 h-16 text-gray-600 mx-auto mb-4" />
-      <h3 className="text-lg font-semibold text-gray-300 mb-2">{title}</h3>
-      <p className="text-gray-400">{description}</p>
+      <h3 className="text-lg font-semibold text-slate-700 mb-2">{title}</h3>
+      <p className="text-slate-500">{description}</p>
     </div>
   );
 };