Clarify web viewer startup path

Author: codex

README-CN.md

@@ -184,13 +184,15 @@ source .venv/bin/activate
 阅读器直接读取仓库中的 `.ipynb` 原文并在前端渲染，不维护额外的网页内容副本。
 
 ```bash
+cd web
 npm install
 npm run dev
 ```
 
 构建并预览静态网站：
 
 ```bash
+cd web
 npm run build
 npm run preview
 ```

README.md

@@ -215,13 +215,15 @@ The reader imports the `.ipynb` files directly and renders them in the browser,
 web content copy.
 
 ```bash
+cd web
 npm install
 npm run dev
 ```
 
 Build and preview the static site:
 
 ```bash
+cd web
 npm run build
 npm run preview
 ```