[Feature Request] i18n support: Simplified Chinese (简体中文) UI localization

Is your feature request related to a problem?

As a Chinese-speaking user, the English-only UI is a barrier to daily use.
While Codex handles Chinese conversation well, menus, settings, and
buttons being English-only reduces accessibility.

Describe the solution you'd like

Add a Language setting (Ctrl+,) with Simplified Chinese (zh-CN) support:

• Localized UI strings for menus, dialogs, tooltips
• Auto-detect OS locale as fallback

Additional context

• China has a large potential user base for AI coding tools
• VS Code, JetBrains IDEs already offer Chinese UI
• Willing to help with translations if a framework is set up