feat: complete E2E deployment flow with storage layered config and values template versioning

- Instance deployment: charts browser, deploy modal, instances list
- Values Template version management (create/history/rollback)
- Storage layered config (cluster > workspace > shared priority)
- Cluster credential decryptIfNeeded for mixed encrypted/plaintext kubeconfig
- YAML syntax validation (client-side + server-side warning)
- Frontend: charts, instances, storage, templates, admin pages
- Backend: storage service, instance service, cluster service, helm client
- Multi-Tenant Kubeconfig.md: added by user

backend/internal/domain/service/cluster_service.go

@@ -175,8 +175,8 @@ func (s *ClusterService) createRestConfig(cluster *entity.Cluster) (*rest.Config
 			kubeconfig = ".kube/config"
 		}
 		// 尝试从文件加载 kubeconfig
-		if _, err := os.Stat(kubeconfig); err == nil {
-			return clientcmd.BuildConfigFromFlags("", kubeconfig)
+		if _, err := os.Stat(kubeconfig); err != nil {
+			return nil, fmt.Errorf("no valid credentials found for cluster %s (no cert/key/token, and kubeconfig file not found: %s)", cluster.Name, kubeconfig)
 		}
 		return clientcmd.BuildConfigFromFlags("", kubeconfig)
 	}