make threshold for setting vtuber isJumping to true configurable

[iccanobif]

Could be a knob like the one we have for moving audio streams across the stereo field, but available on the streamer side. To make things simpler, the threshold could be an attribute in StreamSlotDto and every change could trigger a sendUpdatedStreamSlotState() (debounced, of course). In alternative, we'd need new user-want-to-change-vtuber-threshold and server-vtuber-threshold-changed websocket messages.