Markdown heading syntax (#) is not properly rendered in Feishu interactive
cards. Convert headings to div elements with lark_md format (bold text) for
proper display.
- Add _HEADING_RE regex to match markdown headings (h1-h6)
- Add _split_headings() method to parse and convert headings to div elements
- Update _build_card_elements() to process headings before markdown content
Ahwei