feat(cli): modernize HTML export with interactive charts and filters

- Rewrite export_to_html with Bootstrap 5 styling
- Add Chart.js visualizations (severity, type, category, source)
- Add executive summary dashboard with stat cards
- Add interactive filtering by severity, type, and search
- Add sortable table columns
- Add expandable row details with full finding information
- Add Prism.js syntax highlighting for code snippets
- Display LLM context, confidence, CWE/OWASP, recommendations
- Make responsive with print-friendly CSS
- Update extract_simplified_findings to handle native format
- Update export_to_csv to handle native format with more fields
- Fix export functions to use findings_data instead of sarif_data
- Add safe_escape helper to handle None values

Author: tduhamel42