feat(采购订单): 添加合同条目关联及税率绑定功能

新增采购订单条目与合同条目的关联字段，实现税率和税率锁定的UI绑定
优化采购订单同步逻辑，支持从U8系统获取更多字段信息
调整界面文本显示，修复部分字段绑定问题

client/src/main/java/com/ecep/contract/vm/PurchaseOrderItemViewModel.java

@@ -23,6 +23,11 @@ public class PurchaseOrderItemViewModel extends IdentityViewModel<PurchaseOrderI
      * 关联的库存, Inventory
      */
     private SimpleObjectProperty<Integer> inventory = new SimpleObjectProperty<>();
+    /**
+     * 关联的采购合同条目, ContractItem
+     */
+    private SimpleObjectProperty<Integer> contractItem = new SimpleObjectProperty<>();
+
     private SimpleIntegerProperty refId = new SimpleIntegerProperty();
     private SimpleDoubleProperty quantity = new SimpleDoubleProperty();
     private SimpleDoubleProperty price = new SimpleDoubleProperty();
@@ -36,6 +41,7 @@ public class PurchaseOrderItemViewModel extends IdentityViewModel<PurchaseOrderI
         super.updateFrom(v);
         getOrder().set(v.getOrder());
         getInventory().set(v.getInventoryId());
+        getContractItem().set(v.getContractItemId());
         getRefId().set(v.getRefId());
         getQuantity().set(v.getQuantity());
         getPrice().set(v.getPrice());
@@ -57,6 +63,10 @@ public class PurchaseOrderItemViewModel extends IdentityViewModel<PurchaseOrderI
             v.setInventoryId(inventory.get());
             modified = true;
         }
+        if (!Objects.equals(contractItem.get(), v.getContractItemId())) {
+            v.setContractItemId(contractItem.get());
+            modified = true;
+        }
         if (!Objects.equals(refId.get(), v.getRefId())) {
             v.setRefId(refId.get());
             modified = true;