Fix Marketplace language links

[SivanCola]

Summary

• replace the relative Simplified Chinese README link with an absolute GitHub URL
• update the reciprocal English link for consistent behavior outside the repository view

Root cause

Visual Studio Marketplace resolves the relative ./README.zh-CN.md link against marketplace.visualstudio.com, producing a nonexistent /README.zh-CN.md URL and a 404.

Impact

After the next VSIX is published to Visual Studio Marketplace, the language switch links will work from both GitHub and the Marketplace-rendered README. Extension runtime behavior is unchanged.

Merging this PR alone does not refresh the currently published Marketplace README. Issue #2 should remain open until a release containing this change is published and the live listing is verified.

Validation

• git diff --check
• npm run lint
• npm test (48 passed)
• npm run package:vsix
• npm run verify:vsix-contents
• confirmed both absolute README URLs return HTTP 200
• confirmed the packaged English and Chinese READMEs contain the absolute links
• scanned both README files for remaining relative language links

CI note

GitHub CI currently stops at npm audit --audit-level=high before running its code checks. The audit reports four high-severity transitive vulnerabilities (brace-expansion, fast-uri, js-yaml, and linkify-it) under @vscode/vsce. This PR does not modify package.json or package-lock.json; dependency remediation should be handled separately.

Refs #2