feat(client): add Simplified Chinese dashboard localization

[XJPeng12]

Summary

• Add i18next + react-i18next to the client with English and Simplified Chinese locale files.
• Add a navbar language toggle (EN / 中文) that persists the user's choice in localStorage.
• Translate dashboard UI strings across auth, models, keys, playground, analytics, and premium pages.
• Auto-select zh-CN when the browser language is Chinese on first visit.

Motivation

The dashboard was English-only. This makes the admin UI usable for Chinese-speaking users without relying on browser translation.

Notes

• Dynamic content from the API (model names, provider labels, error messages) is unchanged.
• No server-side changes.

Test plan

• Open the dashboard and switch language via the EN/中文 toggle in the navbar.
• Verify login/setup, keys, models (chat + embeddings), playground, analytics, and premium pages in both locales.
• Refresh the page and confirm the selected language is remembered.
• Run npm run build -w client and confirm the production build succeeds.