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.

web/package.json

@@ -21,6 +21,7 @@
     "@codemirror/commands": "^6.7.1",
     "@codemirror/lang-markdown": "^6.3.1",
     "@codemirror/language": "^6.10.8",
+    "@codemirror/language-data": "^6.5.2",
     "@codemirror/state": "^6.5.1",
     "@codemirror/view": "^6.36.2",
     "@fontsource-variable/jetbrains-mono": "^5.3.0",