Add a red STOP button to abort a run

Operators could not find how to abort a running sequence: the stop control was a small grey icon that read as decoration rather than an emergency control.

• Added a solid red STOP button that morphs into KILL after a stop, with a fixed width so the bottom bar never reflows.
• Added a 500 ms delay before KILL becomes available, so a touchscreen double-tap cannot escalate a graceful stop into a forced kill.
• Fixed the button keeping the previous run's state after a restart or a lost command, and made a kill re-sendable if the run has not ended after 5 seconds.