When a receiver-paired device goes offline, update its battery status
to OFFLINE so the tray and UI show "Battery: 58% (offline)" instead of
the stale "Battery: 58% (discharging)". The last known percentage is
preserved as useful context.
Only applies to devices paired through a receiver (Unifying, Bolt,
Lightspeed) which maintain a persistent connection while available.
Bluetooth devices that disconnect during idle keep their prior status
since going offline is normal power-saving behavior for them.
Add BatteryStatus.OFFLINE (0xFF) as a Solaar-internal value (not a
HID++ protocol value). Fresh battery data from read_battery() replaces
it when the device reconnects.
9ca6e261ab