feat(studio): add English localization

[evanlowe]

Summary

• add a scalable i18next-based localization foundation with Chinese and English catalogs
• add language selection under System Information with persisted locale preference
• localize Studio UI, runtime messages, creation flows, automations, sandbox, and packaged WebUI assets
• add catalog consistency checks and localization regression coverage

Testing

• pre-commit run --all-files
• cd frontend && npx tsc --noEmit
• cd frontend && npm run check:i18n
• cd frontend && npm test
• cd frontend && npm run build
• cd frontend && npm run test:webui-assets