feat(work-orders): 다중 담당자 UI 구현

- actions.ts: createWorkOrder에 assigneeIds 배열 파라미터 추가
- actions.ts: assignWorkOrder가 단일/배열 모두 지원하도록 변경
- WorkOrderCreate.tsx: assigneeIds 배열로 API 전송
- WorkOrderDetail.tsx: 다중 담당자 표시 (쉼표 구분)

src/components/production/WorkOrders/WorkOrderCreate.tsx

@@ -151,7 +151,7 @@ export function WorkOrderCreate() {
         projectName: formData.projectName,
         processType: formData.processType,
         scheduledDate: formData.shipmentDate,
-        assigneeId: formData.assignees.length > 0 ? parseInt(formData.assignees[0]) : undefined,
+        assigneeIds: formData.assignees.map(id => parseInt(id)),
         memo: formData.note || undefined,
       });