feat: Add Chinese (zh_CN) language support for reports

[WinJayX]

• Add messages_zh_CN.properties with complete Chinese translations
• Add Chinese Markdown report template (code-analysis-template_zh_CN.md)
• Update StringManager to support locale-based message loading
• Update MarkdownExporter to use locale-specific templates
• Add zh_CN to supported languages in plugin.properties
• Update frontend to support language selection

This feature enables Chinese users to generate reports in their native language, improving accessibility for the Chinese SonarQube community.

Proposed changes

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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

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• Close #0

Checklist

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• I have read the CONTRIBUTING doc
• I agree with the CODE OF CONDUCT
• Lint and unit tests pass locally with my changes
• SonarCloud and Travis CI tests pass with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

[WinJayX]

⚠️ Important Note about Version Compatibility

Hello maintainers,
This PR was developed based on tag 4.3.0 to add Chinese language support. I noticed that the current dev branch is at version 5.x, which has significant structural changes (e.g., src/main/js/components/ has been moved to src/main/js/global_page/components/).
Due to these differences, there are merge conflicts that I cannot resolve directly.

Options for maintainers:

1. Create a maintenance branch for 4.x - If you still maintain the 4.x line, I can rebase my changes onto a maintenance/4.x branch.
2. Port to 5.x - If you prefer, I can study the 5.x codebase and submit a new PR adapted for the current architecture.
3. Manual merge - You can manually cherry-pick the i18n-related changes into 5.x if that's easier.

What this PR includes:

• Complete Chinese (zh_CN) translations (messages_zh_CN.properties)
• Chinese Markdown report template
• Locale-aware message loading in StringManager.java
• Template selection logic in MarkdownExporter.java
  I'm happy to assist with whatever approach works best for the project. Please let me know how you'd like to proceed.
  Thank you for your time! 🙏