feat: 增强数据集预览功能

重构数据集预览页支持多文件切换与内容高亮，Mock 新增预览数据集与按文件 ID 路由的内容返回，并补充回归测试脚本与 npm 脚本注册。

frontend/src/mock/adapter.ts

@@ -12,6 +12,7 @@ import {
   mockTrainedModels,
   mockLocalModels,
   mockDatasets,
+  mockDatasetPreviews,
   mockFineTuneList,
   mockCompareList,
   mockEvalList,
@@ -129,7 +130,12 @@ async function handleMock(config: AxiosRequestConfig) {
   m = url.match(/^\/dataset-manage\/upload\/([^/]+)$/)
   if (m && method === 'post') return ok({ uploaded: true })
   m = url.match(/^\/dataset-manage\/preview\/([^/]+)$/)
-  if (m && method === 'get') return ok(mockLogContent)
+  if (m && method === 'get') {
+    const fileId = decodeURIComponent(m[1])
+    const fallbackKey = fileId.endsWith('-readme') ? 'mock-readme' : 'mock-jsonl'
+    const content = mockDatasetPreviews[fileId] ?? mockDatasetPreviews[fallbackKey]
+    return ok({ content })
+  }
 
   // ==================== 训练任务 ====================
   if (url === '/fine-tune' && method === 'get') return ok(mockFineTuneList)
@@ -265,4 +271,4 @@ export function installMockAdapter(instance: AxiosInstance) {
       return fail(e.message || 'Mock 错误', 500, config)
     }
   }
-}
+}