Add Vietnamese (vi) translation

[needsbuilder]

Summary

Add Vietnamese language support to EasyClaw.

Suggested Scope

1. Create src/renderer/src/i18n/locales/vi.json based on existing locale files
2. Create README.vi.md based on README.md
3. Update language selector links in all README files
4. Register the new locale in the i18next configuration

Why This Is a Good First Issue

• Translation work doesn't require deep codebase knowledge
• Existing locale files serve as a clear template
• Great way to contribute to an open source project