feat: integrate HTML report into library and its tests

[patelheet30]

Summary

Adds HTML export functionality to LintData with professional visual styling.

New Features

✅ HTML Report Generation

• Beautiful gradient design with modern styling
• Color-coded severity levels (high/medium/low)
• Responsive layout for all screen sizes
• Metadata display (rows, columns, issue count)

✅ File Export

• Save HTML reports to file with output parameter
• UTF-8 encoding for special characters
• Automatic HTML escaping for security

✅ Easy to Use

# Generate HTML report
html = df.lint.report(format='html')

# Save to file
df.lint.report(format='html', output='report.html')

Documentation

• CHANGELOG.md updated
• README.md roadmap updated
• Method docstrings updated with examples

Breaking Changes

None - fully backward compatible. Default format remains 'text'.

[Copilot]

Pull request overview

This PR adds HTML report generation capabilities to the LintData library, enabling users to export data quality reports in a visually styled HTML format alongside the existing text format.

Key changes:

• Added new HTMLReportFormatter class for generating styled HTML reports with color-coded severity levels
• Extended report() method with format and output parameters for flexible report generation
• Comprehensive test suite with 15 test cases covering various scenarios

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
tests/test_html_report.py	New test file with 15 test cases covering HTML report generation, file output, metadata display, severity levels, and edge cases
src/lintdata/report_formatter.py	New formatter class that generates styled HTML reports with responsive design, severity-based color coding, and proper HTML escaping
src/lintdata/accessor.py	Enhanced report() method to support HTML format via new format and output parameters while maintaining backward compatibility
README.md	Updated roadmap to mark HTML Reports feature as completed
CHANGELOG.md	Documented new HTML reporting features and API changes

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