[main][ADD][i18n] Polytalk: Added Chinese translation UI

[bizzappdev]

Summary

Adding Chinese language support for the PolyTalk CE UI.

• Added zh.json translation file in app/locales/ directory
• Integrated Chinese (zh) locale into the existing i18n translation framework
• All UI strings translated to Chinese following the same JSON structure as existing locale files

Testing

• pre-commit run --all-files
• pytest tests/ -v
• Manual Docker and browser testing,

Checklist

• I did not commit secrets, generated media, downloaded voices, or local config.
• Documentation was updated for user-facing or deployment-facing changes.
• Logs are appropriate for the configured log level.