i18n: Move translations to `ui/public` and enable dynamic loading to reduce bundle size

[choo121600]

Summary

• Moved all translation (i18n) files from ui/src/i18n/locales to ui/public/i18n/locales.
• Adopted i18next-http-backend to load translation files via HTTP dynamically.
• Updated FastAPI static file serving logic to ensure translation files are accessible in both dev and prod environments.
  • Unified i18next loadPath to /static/i18n/locales/{{lng}}/{{ns}}.json for both environments.
• Improved maintainability and scalability of the i18n system.

Details

• i18next-http-backend allows translation files to be loaded on demand, reducing the initial bundle size and supporting dynamic language/namespace loading.
• FastAPI dev mode now mounts the public i18n directory to /static/i18n, ensuring translation files are served correctly during development.
• Production mode continues to serve translation files from the built dist directory.
• The i18n config (config.ts) was updated to use a unified HTTP path for translation files, simplifying the frontend logic.

Breaking Changes

• If we have custom scripts or tools that expect translation files in src, please update them to use the new location in public/i18n/locales.

closes: #51123

Opened a new PR to avoid potential issues with translation file conflicts in the previous one. Sorry for the inconvenience.

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