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/layout/SidebarLayout/SidebarLayout.tsx

@@ -13,7 +13,7 @@ export default function SidebarLayout({ items, children }: SidebarLayoutProps) {
   return (
     <div className="relative min-h-screen flex bg-dark text-primary overflow-hidden">
       <div
-        className="pointer-events-none absolute inset-0 bg-app-gradient opacity-90"
+        className="pointer-events-none absolute inset-0 bg-app-gradient"
         aria-hidden="true"
       />
       <SidebarNav