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

- [x] I have searched the [issues](https://github.com/ActivityWatch/activitywatch/issues) of this repo and believe that this is not a duplicate.

- [x] I have searched the [documentation](https://docs.activitywatch.net/en/latest/) and believe that my question is not covered.

## 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!