Add i18n support for UI text (English / Chinese switching)

[FatBy]

Description

Currently DD-OS UI text is a mix of Chinese and English hardcoded in components. We should extract all UI strings into a centralized i18n system so users can switch between languages.

Current Behavior

• Button labels, placeholder text, and section headers are hardcoded in Chinese
• No way for English-speaking users to use the interface

Expected Behavior

• A language toggle in Settings
• All UI text loaded from language files (e.g., locales/zh.json, locales/en.json)

Suggested Approach

1. Create src/locales/zh.json and src/locales/en.json
2. Use react-i18next or a simple context-based solution
3. Replace hardcoded strings in components with translation keys
4. Add language toggle to Settings panel

Tech Stack

• React 18, TypeScript, Zustand for state