xterm measures its cell at open() and only re-measures on a fontFamily/
fontSize change or a grid resize. JetBrains Mono is font-display: swap and
usually still loading when /chat mounts, so the grid kept fallback metrics
until a sidebar toggle resized the host. Wait on document.fonts for the
bundled faces, then re-measure, refit, and clear the WebGL atlas. Applied
to the /chat terminal and the Hermes Console modal.
Fixes#92899