docs: add FAQ documentation for common issues (#544)

- Add FAQ.md in English, Chinese, and Japanese
- Link FAQ from each language README
- Cover: PDF export, offline deployment, self-hosted models, image upload

docs/cn/README_CN.md

@@ -42,6 +42,7 @@ https://github.com/user-attachments/assets/b2eef5f3-b335-4e71-a755-dc2e80931979
   - [多提供商支持](#多提供商支持)
   - [工作原理](#工作原理)
   - [支持与联系](#支持与联系)
+  - [常见问题](#常见问题)
   - [Star历史](#star历史)
 
 ## 示例
@@ -238,6 +239,10 @@ npm run dev
 
 -   邮箱：me[at]jiang.jp
 
+## 常见问题
+
+请参阅 [FAQ](./FAQ.md) 了解常见问题和解决方案。
+
 ## Star历史
 
 [![Star History Chart](https://api.star-history.com/svg?repos=DayuanJiang/next-ai-draw-io&type=date&legend=top-left)](https://www.star-history.com/#DayuanJiang/next-ai-draw-io&type=date&legend=top-left)