[Bug] Safely handle multiple host connections #2551
Description
Since we introduced USB support, it is possible to have two Pybricks Code instances connected. (This is a good thing; more on that later.)
Since loading a big program via Bluetooth takes a while, it is possible to press the run button on the other connections and get things in a bad state.
Related: we need a similar protection for when two hosts start sending a program at the same time.
Originally posted by @laurensvalk in #2383