i18n: Add Chinese language variants

[etnperlong]

Summary

This PR adds support for three Chinese language variants:

• Simplified Chinese (zh-Hans)
• Traditional Chinese (zh-Hant)
• Cantonese, Hong Kong (yue-Hant-HK)

It also updates the language selector and i18n configuration so Chinese browser language codes are detected and
normalized correctly.

Changes

• Add Chinese translation files for zh-Hans, zh-Hant, and yue-Hant-HK
• Register the new locales in the i18n resource configuration
• Add the new language options to the top menu language dropdown
• Configure browser language detection options
• Add Chinese-specific fallback language mappings
• Normalize detected Chinese language codes such as zh-CN, zh-TW, and zh-HK

Verification

• Confirmed all new translation files match the English locale key structure
• Confirmed the app builds successfully with yarn build
• Manually checked the Chinese UI text and language behavior

AI assistance disclosure

This PR was implemented with AI coding assistance. I am a native Cantonese, Chinese speaker, and I manually reviewed both the
code changes and the Chinese translations before submitting this PR.