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/EmptyState.tsx

@@ -24,11 +24,11 @@ export const EmptyState: React.FC<EmptyStateProps> = ({
   action,
 }) => {
   return (
-    <div className="text-center py-16 bg-gray-800/30 border border-gray-700 border-dashed rounded-lg">
+    <div className="text-center py-16 bg-white border border-slate-200 border-dashed rounded-lg">
       {Icon && <Icon className="w-16 h-16 mx-auto mb-4 text-gray-600" />}
-      <h3 className="text-lg font-semibold text-gray-400 mb-2">{title}</h3>
+      <h3 className="text-lg font-semibold text-slate-500 mb-2">{title}</h3>
       {description && (
-        <p className="text-sm text-gray-500 mb-6">{description}</p>
+        <p className="text-sm text-slate-500 mb-6">{description}</p>
       )}
       {action && (
         <Button

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>
   );
 };

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

@@ -23,14 +23,14 @@ export const LoadingState: React.FC<LoadingStateProps> = ({
 }) => {
   const content = (
     <div className="flex flex-col items-center justify-center gap-4">
-      <div className={`${sizeStyles[size]} border-purple-600 border-t-transparent rounded-full animate-spin`} />
-      {message && <p className="text-sm text-gray-400">{message}</p>}
+      <div className={`${sizeStyles[size]} border-blue-600 border-t-transparent rounded-full animate-spin`} />
+      {message && <p className="text-sm text-slate-500">{message}</p>}
     </div>
   );
 
   if (fullScreen) {
     return (
-      <div className="fixed inset-0 flex items-center justify-center bg-gray-900/80 backdrop-blur-sm z-50">
+      <div className="fixed inset-0 flex items-center justify-center bg-white/80 backdrop-blur-sm z-50">
         {content}
       </div>
     );
@@ -42,4 +42,3 @@ export const LoadingState: React.FC<LoadingStateProps> = ({
     </div>
   );
 };
-