🌐 Internationalization (i18n)

[coder-ralph]

• Add multi-language support to the UI (starting with Chinese)
• Allow users to switch languages easily
• Prepare the app for future translations

Scope

• Add i18n framework (e.g., next-intl)
• Extract UI strings into translation files
• Add language switcher in UI
• Support at least:
  • English (default)
  • Simplified Chinese (简体中文) (zh-CN)
  • Traditional Chinese (繁體中文) (zh-TW)
• Optional: auto-detect browser language