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.
/ is now the swarm's name over the digital-rain backdrop the hive
dashboard's home has, and the swarm name in the header links to it. The
hive directory moves to /hives, where the nav's hives tab points. The
swarm name is read once per page load by a shared hook, the header and
the start page both use it. Closes#5252
