board: a card turns over to a checklist, labels and a description

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.

migrations/0025_card_checklist.sql

@@ -0,0 +1,8 @@
+-- What a card holds besides its title, the way a card on a paper board does:
+-- a checklist of the steps it takes, and labels that say what kind of work
+-- it is. Both are small, read with the card and written with it, so they are
+-- JSON on the card rather than tables of their own: `checklist` is an array
+-- of `{"text": …, "done": …}`, `labels` an array of names. The description is
+-- the card's `body`, which it has had from the start.
+ALTER TABLE cards ADD COLUMN checklist TEXT NOT NULL DEFAULT '[]';
+ALTER TABLE cards ADD COLUMN labels TEXT NOT NULL DEFAULT '[]';