test: add comprehensive tests for franc language detection
- Add unit tests for getLanguage function with franc
- Test input validation, language detection for 10+ languages
- Add critical moderation tests for English vs French 'retard' disambiguation
- Verify franc can distinguish French 'delay' from English slur usage
- Include edge cases: emojis, special chars, mixed languages
- Add vitest configuration and test scripts
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Co-Authored-By: Claude <noreply@anthropic.com>