Editor: quantities inside {} are not highlighted

[dubadub]

Found while reviewing the fix for #490.

static/js/src/cooklang-mode.js returns "measurement" for the quantity part of {200%g} (and "unit" only after %). measurement is not a Lezer highlight tag, so CodeMirror logs "Unknown highlighting tag measurement" and the number gets no class or colour; only the unit is styled.

Fix: map the quantity to a real tag (e.g. t.number) and give it a .cm-cook-quantity class in static/css/input.css on a token colour.