Add browser locale redirect for website

[Prodesire]

Summary

• add a Docusaurus client module that redirects first-time root visits to the browser's preferred non-default locale
• map browser language variants such as zh-CN, ja-JP, and pt-BR to configured Docusaurus locales
• add Node tests for locale selection and root-only redirect behavior

Verification

• npm test
• npm run typecheck
• npm run build
• Playwright smoke test against local build: zh-CN -> /iac-code/zh-Hans/, en-US -> /iac-code/