1) Layout: a top strip shows training time | temperature | clock ('external'
things); the bottom panel row is now HR, distance, speed, altitude (bigger);
the separate HR bar is gone; the three controls (HR toggle, pause, stop)
share the width equally instead of clustering right.
2) Clear currentHr/currentBleData at recording start (+ ble_clear_hr) so a
stale bpm from the previous session no longer hangs on the display or gets
written onto this session's points; show -- until a fresh reading.