feat: add desktop i18n and zh-CN translations

[huangcancan-xbc]

Summary

• add a lightweight i18n provider for the desktop app
• introduce zh-CN translations and language toggle in settings
• translate key desktop screens (settings + main recorder UI)

Notes

• setup/editor/screenshot-editor remain in English for now
• formatting not run (repo-wide biome currently reports existing errors)

[jiasongji]

Thanks for working on the Chinese localization! I’m a native Chinese speaker and I’d be happy to help review or contribute additional translations, especially for the setup flow, editor, and screenshot-editor views. Let me know if there’s anything I can assist with.

[huangcancan-xbc]

感谢您参与中文本地化工作！我是中文母语者，很乐意帮忙审校或补充翻译，特别是设置流程、编辑器和截图编辑器视图方面的翻译。如有任何需要我帮忙的地方，请随时联系我。

Thank you so much for your kindness and help! 😊

I'm primarily a C++ developer and not very familiar with the programming language and the complete internationalization process of this project. I really wanted a Chinese version of Cap, so I decided to give it a try and submitted this PR. Many parts were done quickly as I learned along the way, so there are many areas that could be improved.

Currently, I'm still focusing on improving my technical skills, and it's difficult for me to dedicate much time to in-depth research. However, I would be very grateful if you could help review the translation or provide suggestions for improvement – ​​especially regarding the setup process, the editor, and the screenshot editor interface that you mentioned!

In the future, once I've made more progress in my studies, I would be happy to have a more in-depth discussion and collaboration with you. Thank you again for your support!

(非常感谢你的热情！如果有翻译问题欢迎直接指出或补充～未来有缘深入交流！)