[medium] Migrate hardcoded zh-CN strings to react-i18next

[Do-fei]

Every UI string and most error messages are hardcoded Chinese literals (e.g. Settings.tsx, Home.tsx, error messages thrown from server/db.ts:48 / server/routers.ts:168, 282).

Fix: add react-i18next with zh-CN + en locales; pass i18n keys through tRPC errors.