The rain read only the OS reduced-motion query, once, at mount. The settings
menu's motion override (data-motion on the document) wins over the OS in
either direction everywhere else, and the shell applies it from an effect
that runs after this one. Whether the rain may move is now rainAllowed(), and
the component re-evaluates it on mount, on every document attribute change
and on the media query's change, starting or stopping the interval and
clearing the canvas.
The rain's column maths and fade colour move from MatrixRain into
shell/rain.ts, and the request-once swarm name loader from useSwarmName
into shell/swarm-name.ts. Both are plain functions with their input
passed in, so tests cover them; the components keep the DOM and canvas
work.
