fix: Change purchase orders API parameter from 'offset' to 'skip' to match backend

Frontend was using 'offset' parameter but backend expects 'skip', causing
the parameter to be ignored and potentially contributing to empty results.

services/procurement/app/api/purchase_orders.py uses 'skip' parameter.

frontend/src/api/services/purchase_orders.ts

@@ -112,7 +112,7 @@ export interface PurchaseOrderSearchParams {
   date_to?: string; // YYYY-MM-DD
   search_term?: string;
   limit?: number;
-  offset?: number;
+  skip?: number;  // ✅ Changed from "offset" to "skip" to match backend
 }
 
 export interface PurchaseOrderUpdateData {