Replaces the segmented An/Aus toggle with a single button that always
shows the action a click would perform: printer on -> button offers
"Aus" (red), printer off -> button offers "An" (green). Matches how
the physical switch actually works (only one direction is ever
possible at a time) instead of presenting two options where one is
always a no-op.
_headerPowerState tracks the last known actual state so
toggleHeaderPower() knows which action to send; unrelated printer-tab
button/other UI untouched.
viewit
e70720bc27