Proposal: Add Chinese (zh-CN) documentation / README translation

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Issue

Hi, thank you for the great project!

I would like to contribute a Simplified Chinese (zh-CN) translation to help more users in China understand and use ActivityWatch.

My initial plan is:

• Add a README.zh-CN.md that is a Chinese translation of the main README.md.
• Add a short language switch section to the top of README.md, e.g.:
  • Read this in other languages: [简体中文](./README.zh-CN.md)
• Optionally, if you think it makes sense, I can also help translate some key pages of the documentation site (such as Getting started / FAQ) and follow whatever structure you prefer for multilingual docs.

Before I start working on this, I would like to confirm:

1. Are you open to having a Chinese translation in this repository?
2. Do you have any preference on:
   • The file name (e.g. README.zh-CN.md vs README_zh-CN.md)?
   • Where translations should live (root vs a docs/ or i18n/ subdirectory)?
3. Are there any existing terminology/style guidelines that I should follow when translating?

If you agree with this direction, I will:

• Fork the repository.
• Create a new branch for the zh-CN translation.
• Submit a pull request referencing this issue.

Thanks again for your time, and looking forward to your feedback!