Enter on a card floats a sheet over the board: its labels, a markdown
description written in the vim editor (:w writes it to the card), and a
checklist walked with j/k, ticked with Space, grown with o. Labels take
their colour from their name. A repeating card comes back with its
checklist reopened.
Code in the editor and in rendered fences is coloured from nvim's
yorha.lua, one class per highlight group, for every language
@codemirror/language-data knows; the languages load on first use.