Fix visualization HTML charset handling

[Samay10]

Fixes #234

Description

Saved visualization HTML should always be interpreted as UTF-8. Without an explicit charset, some browsers can guess a different encoding based on the user’s system settings that might cause non-ASCII text to render incorrectly.

This change adds a UTF-8 charset meta tag to the HTML returned by visualize(), so saved visualization files render consistently across browsers.

How Has This Been Tested?

• Ran pyink langextract/visualization.py tests/visualization_test.py --config pyproject.toml
• Ran pytest tests/visualization_test.py (5 passed)
• Ran pytest tests; the visualization tests passed, but the full suite still has unrelated local failures from the missing optional openai package and one pip-install plugin discovery test

Checklist

• My code follows the style guidelines
• I have self-reviewed my own code
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests passed locally with my changes

[github-actions]

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