修复：替换讲解详情无封面占位图

src/utils/placeholders.ts

@@ -1,2 +1,3 @@
 export const EXHIBIT_PLACEHOLDER_IMAGE = '/static/icons/marker-exhibit.svg'
+export const EXPLAIN_DETAIL_PLACEHOLDER_IMAGE = '/static/explain/exhibit-detail-placeholder.jpg'
 export const HALL_PLACEHOLDER_IMAGE = '/static/icons/marker-hall.svg'

src/view-models/explainViewModels.ts

@@ -6,7 +6,7 @@ import type {
   SearchIndexItem
 } from '@/domain/museum'
 import {
-  EXHIBIT_PLACEHOLDER_IMAGE,
+  EXPLAIN_DETAIL_PLACEHOLDER_IMAGE,
   HALL_PLACEHOLDER_IMAGE
 } from '@/utils/placeholders'
 
@@ -185,7 +185,7 @@ export const toExplainDetailPageViewModel = (exhibit: MuseumExhibit): ExplainDet
     subtitle: buildSubtitle(exhibit),
     contentType: contentTypeFor(exhibit),
     coverImages: [
-      exhibit.image || EXHIBIT_PLACEHOLDER_IMAGE,
+      exhibit.image || EXPLAIN_DETAIL_PLACEHOLDER_IMAGE,
       ...(exhibit.galleryUrls || [])
     ].filter(Boolean),
     hallId: exhibit.hallId,