feat(i18n): add app language switching and Chinese docs

[guozhijian611]

Summary

• Add persisted app language preference with system, en, and zh-CN options.
• Add i18n provider/hooks and translate major app surfaces, including settings, composer, Git, files, messages, home, mobile setup, update toasts, terminal, and layout controls.
• Add a Simplified Chinese README and localization guide.

Validation

• npm run typecheck
• npm run test
• cd src-tauri && cargo check

Note: cargo check only reports existing dead-code warnings.