Add a lightweight i18n system to the web dashboard with English (default) and
Chinese language support. A language switcher with flag icons is placed in the
header bar, allowing users to toggle between languages. The choice persists
to localStorage.
Implementation:
- src/i18n/ — types, translation files (en.ts, zh.ts), React context + hook
- LanguageSwitcher component shows the *other* language's flag as the toggle
- I18nProvider wraps the app in main.tsx
- All 8 pages + OAuth components updated to use t() translation calls
- Zero new dependencies — pure React context + localStorage
