refactor: improve HTMLFormatter code readability and maintainability

- Extract all inline CSS styles to named class constants
- Split large methods into smaller, focused helper methods
- Add reusable helper methods for common patterns:
  - _skip_whitespace() for navigating inline blocks
  - _is_bullet_marker() for identifying bullet markers
  - _extract_bullet_parts() for parsing bullet list lines
  - _format_bullet_list_item() for formatting list items
- Split block formatting into dedicated methods:
  - _format_header_block(), _format_code_block(), _format_divider_block()
  - _format_link_block(), _format_inline_code_block(), _format_mention_block()
  - _format_table_header(), _format_table_body(), _format_table_row()
  - _build_expandable_summary() for expandable block arrows

No functional changes - all 36 HTML format tests passing
All pre-commit checks passing (black, isort, flake8, mypy)

Author: MikaKerman