fix: save item position after drag-and-drop reordering

src/admin/pages/OfferDetail.tsx

@@ -408,7 +408,7 @@ export default function OfferDetail() {
       const response = await apiFetch(url, {
         method: isEdit ? 'PUT' : 'POST',
         headers: { 'Content-Type': 'application/json' },
-        body: JSON.stringify({ ...form, items, sections: sections.map((s, i) => ({ ...s, position: i })) })
+        body: JSON.stringify({ ...form, items: items.map((item, i) => ({ ...item, position: i })), sections: sections.map((s, i) => ({ ...s, position: i })) })
       })
       const result = await response.json()
       if (result.success) {