When a list-handler handoff is deferred waiting for the 5h usage limit to
reset, the Mission Control pane title is augmented with ⏳ waiting for
limit reset (HH:mm) so the wait is visible at a glance. Extends
HubBroadcaster.HandoffRequested with an optional deferredUntil DateTime
parameter, wired through SignalR to MissionControlViewModel, which formats
it into the pane title via Loc.T(missionControl.waitingForLimitReset).
