docs: add FAQ section to README

- Add FAQ covering storage privacy and sync capacity limits
- Update both Chinese (README.md) and English (README_EN.md)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: ruibin3

README.md

@@ -100,6 +100,27 @@ npm run build:prod
 
 ---
 
+## ❓ 常见问题
+
+### 笔记保存在哪里？安全吗？
+
+SlideNote 使用 **Chrome Storage API** 存储数据：
+
+- ✅ 不经过第三方服务器，数据直接在你本地和 Google 之间同步
+- ✅ 项目已开源，代码透明
+- ✅ 不收集任何个人信息
+
+### 同步容量有限制吗？
+
+Chrome Storage Sync API 有以下限制：
+
+- 总容量约 100KB（约 5 万汉字）
+- 适合存储：API Key、命令、提示词等碎片信息
+
+SlideNote 的定位是「便利贴」，不是「笔记本」。如需存储大量内容，建议使用 Notion、Obsidian 等专业笔记软件。
+
+---
+
 ## 🛠️ 技术栈
 
 ```

README_EN.md

@@ -100,6 +100,27 @@ npm run build:prod
 
 ---
 
+## ❓ FAQ
+
+### Where are notes stored? Is it secure?
+
+SlideNote uses **Chrome Storage API** to store data:
+
+- ✅ No third-party servers; data syncs directly between your local and Google
+- ✅ Open source, code is transparent
+- ✅ No personal information collected
+
+### Is there a sync limit?
+
+Chrome Storage Sync API has these limits:
+
+- Total capacity ~100KB (about 50,000 Chinese characters)
+- Suitable for: API Keys, commands, prompts, and similar fragmented information
+
+SlideNote is a "sticky note", not a "notebook". For storing large amounts of content, consider using dedicated note apps like Notion or Obsidian.
+
+---
+
 ## 🛠️ Tech Stack
 
 ```