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/package.json

@@ -11,6 +11,7 @@
     "test:data-process-list": "node scripts/regression-data-process-list.mjs",
     "test:data-process-wizard": "node scripts/regression-data-process-wizard.mjs",
     "test:dataset-task-tab": "node scripts/regression-dataset-task-tab.mjs",
+    "test:dataset-preview": "node scripts/regression-dataset-preview.mjs",
     "test:model-manage": "node scripts/regression-model-manage.mjs",
     "test:training-log-layout": "node scripts/regression-training-log-layout.mjs",
     "test:page-surface": "node scripts/regression-page-surface.mjs"