Implement a "Cancel Purchase Order" feature, replacing the existing "Delete Purchase Order" behaviour across the stack. Requirements:

BACKEND
1. Rename the operation from DeletePurchaseOrder to CancelPurchaseOrder everywhere it appears (application service interface + base service, WebAPI controller action AND its HTTP route, WebApp MVC controller, WebApp service + interface).
2. Change the cancel behaviour: instead of soft-deleting via entity.IsActive = false, set entity.Enum_Status = (int)PurchaseOrderStatus.Cancelled (PurchaseOrderStatus is in BMP.SharedInterfaces.Enums.BMPBase — add the using if needed).
3. Update user-facing/log text from delete/deleting to cancel/cancelling.

FRONTEND
4. On the list script rename deletePurchaseOrder url to cancelPurchaseOrder, btnDeletePO to btnCancelPO, onBtnDeleteClick to onBtnCancelClick, and delete/deleted messages to cancel/cancelled.

Do NOT rename unrelated operations that merely share a name prefix (e.g. DeletePurchaseOrderLines is a different operation and must stay). Keep unrelated code unchanged.
