feat(frontend): add Helm chart browser, monitoring, chart-references and values templates pages

Add new frontend pages for the multi-tenant OCDP platform:

- Charts page (/charts): Browse Harbor OCI registries to list Helm chart repositories
  and versions, with deploy modal to launch charts on selected clusters
- Monitoring page (/monitoring): Display cluster metrics (CPU/Memory/GPU usage)
  and per-node details with resource utilization bars
- Chart References page (/chart-references): CRUD for chart metadata references
- Values Templates page (/templates): CRUD for Helm values templates with version
  history and rollback support
- Sidebar: Add Charts navigation, update Storage and Templates links
- api.ts: Add all API client functions (clusterApi, registryApi, instanceApi,
  monitoringApi, storageApi, chartReferenceApi, valuesTemplateApi,
  workspaceApi, userApi) with full TypeScript types

Note: deploy flow and values template rollback not yet end-to-end tested.

backend/internal/domain/entity/errors.go

@@ -37,4 +37,32 @@ var (
 	ErrArtifactNotFound     = errors.New("artifact not found")
 	ErrRepositoryNotFound   = errors.New("repository not found")
 	ErrValuesSchemaNotFound = errors.New("values schema not found")
+	ErrValuesNotFound       = errors.New("values not found")
+
+	// Workspace errors
+	ErrInvalidWorkspaceName = errors.New("invalid workspace name")
+	ErrWorkspaceNotFound    = errors.New("workspace not found")
+	ErrWorkspaceExists      = errors.New("workspace already exists")
+
+	// Quota errors
+	ErrQuotaExceeded      = errors.New("quota exceeded")
+	ErrInvalidQuota       = errors.New("invalid quota")
+
+	// Storage errors
+	ErrInvalidStorageName   = errors.New("invalid storage name")
+	ErrStorageNotFound      = errors.New("storage not found")
+	ErrStorageExists        = errors.New("storage already exists")
+
+	// Chart Reference errors
+	ErrInvalidChartReferenceName = errors.New("invalid chart reference name")
+	ErrChartReferenceNotFound    = errors.New("chart reference not found")
+	ErrChartReferenceExists      = errors.New("chart reference already exists")
+
+	// Template errors
+	ErrInvalidTemplateName = errors.New("invalid template name")
+	ErrTemplateNotFound    = errors.New("template not found")
+	ErrTemplateExists      = errors.New("template already exists")
+
+	// Permission errors
+	ErrPermissionDenied = errors.New("permission denied")
 )