Summary

• Add Vietnamese and English localization across the frontend.
• Add a persistent language switcher that preserves the selected locale after reload.
• Move UI text, validation messages, errors, empty states, loading states, modals, notifications, and pagination labels to translation resources.
• Format currencies, numbers, and dates using vi-VN and en-US.
• Keep API values, IDs, currency codes, roles, and business enums unchanged.
• Localize document titles and accessibility labels.
• Group theme and language controls in a shared header control area.
• Reserve space for Zalo's native right-buttons to prevent the language switcher from being obscured.
• Document the i18n and header layout conventions in project memory.