feat: 完成全部接口替换为新后端接口

miniprogram/pages/category/category.model.ts

@@ -1,3 +1,4 @@
+import { resolvePreviewUrl } from '../../config/assets';
 import type { ApiCategory, ApiWorksheet } from '../../services/content-api';
 
 export interface CategoryTab {
@@ -95,7 +96,7 @@ export function buildCategoryDataset(
             title: row.title,
             subtitle: row.subtitle,
             icon: icons.get(row.category) || '📄',
-            previewImg: row.previewImg || '',
+            previewImg: resolvePreviewUrl(row.previewImg),
             ageBand: `${row.ageMin}-${row.ageMax}岁`,
             difficultyLabel: DIFFICULTY_LABELS[row.difficulty - 1] || '基础',
             path: row.path,