Adds code to requeest global setting from server

- Settings are individually cached for 5 minutes
- For now, use it for the purchase order reference prefix

lib/widget/purchase_order_list.dart

@@ -55,9 +55,14 @@ class _PaginatedPurchaseOrderListState extends PaginatedSearchState<PaginatedPur
 
   _PaginatedPurchaseOrderListState(Map<String, String> filters) : super(filters);
 
+  // Purchase order prefix
+  String _poPrefix = "";
+
   @override
   Future<InvenTreePageResponse?> requestPage(int limit, int offset, Map<String, String> params) async {
 
+    _poPrefix = await InvenTreeAPI().getGlobalSetting("PURCHASEORDER_REFERENCE_PREFIX");
+
     params["outstanding"] = "true";
 
     final page = await InvenTreePurchaseOrder().listPaginated(limit, offset, filters: params);
@@ -72,9 +77,9 @@ class _PaginatedPurchaseOrderListState extends PaginatedSearchState<PaginatedPur
     InvenTreePurchaseOrder order = model as InvenTreePurchaseOrder;
 
     InvenTreeCompany? supplier = order.supplier;
-
+    
     return ListTile(
-      title: Text(order.reference),
+      title: Text("${_poPrefix}${order.reference}"),
       subtitle: Text(order.description),
       leading: supplier == null ? null : InvenTreeAPI().getImage(
         supplier.thumbnail,