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

README.md

@@ -45,6 +45,7 @@ https://github.com/user-attachments/assets/9d60a3e8-4a1c-4b5e-acbb-26af2d3eabd1
   - [Multi-Provider Support](#multi-provider-support)
   - [How It Works](#how-it-works)
   - [Support \& Contact](#support--contact)
+  - [FAQ](#faq)
   - [Star History](#star-history)
 
 ## Examples
@@ -246,6 +247,10 @@ For support or inquiries, please open an issue on the GitHub repository or conta
 
 -   Email: me[at]jiang.jp
 
+## FAQ
+
+See [FAQ](./docs/en/FAQ.md) for common issues and solutions.
+
 ## Star History
 
 [![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)