v0.2.3

UI polish and notebook editor improvements:

• Notebook cells with Monaco syntax highlighting (Python, SQL, Markdown)
• Markdown cells render formatted with edit/preview toggle
• Table of contents, drag reorder, resizable width
• Dashboard widget: color encoding pivot, date grouping fix, no default LIMIT
• Overview: auto-refresh, sorted pipeline health, consistent card styling
• Output panel: collapsible with persistent state
• Exact row counts, Hide Stats, seed color in DAG
• Accessibility: ARIA labels across all panels
• python -m havn support