fix: [stocks] Server Action 직렬화 오류 수정 (undefined → null)

- Next.js Server Actions는 undefined 직렬화 불가
- bendingLot: undefined → null
- rawLotNo/fabricLotNo/material: undefined → null (via ??)
- itemId: undefined → null

src/components/stocks/actions.ts

@@ -104,15 +104,15 @@ export interface StockOrder {
     prodCode: string;
     specCode: string;
     lengthCode: string;
-    rawLotNo?: string;
-    fabricLotNo?: string;
-    material?: string;
-  };
+    rawLotNo?: string | null;
+    fabricLotNo?: string | null;
+    material?: string | null;
+  } | null;
 }
 
 export interface StockOrderItem {
   id: string;
-  itemId?: number;
+  itemId?: number | null;
   itemCode: string;
   itemName: string;
   specification: string;
@@ -209,17 +209,17 @@ function transformApiToFrontend(apiData: ApiStockOrder): StockOrder {
       prodCode: bendingLotData.prod_code || '',
       specCode: bendingLotData.spec_code || '',
       lengthCode: bendingLotData.length_code || '',
-      rawLotNo: bendingLotData.raw_lot_no,
-      fabricLotNo: bendingLotData.fabric_lot_no,
-      material: bendingLotData.material,
-    } : undefined,
+      rawLotNo: bendingLotData.raw_lot_no ?? null,
+      fabricLotNo: bendingLotData.fabric_lot_no ?? null,
+      material: bendingLotData.material ?? null,
+    } : null,
   };
 }
 
 function transformItemApiToFrontend(apiItem: ApiStockOrderItem): StockOrderItem {
   return {
     id: String(apiItem.id),
-    itemId: apiItem.item_id ?? undefined,
+    itemId: apiItem.item_id ?? null,
     itemCode: apiItem.item_code || '',
     itemName: apiItem.item_name,
     specification: apiItem.specification || '',