ui: Show offline status for receiver-paired device batteries (#3217)

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.

lib/logitech_receiver/common.py

@@ -645,6 +645,8 @@ class BatteryStatus(Flag):
     SLOW_RECHARGE = 0x04
     INVALID_BATTERY = 0x05
     THERMAL_ERROR = 0x06
+    # Solaar internal — not a HID++ protocol value
+    OFFLINE = 0xFF
 
 
 class BatteryLevelApproximation(IntEnum):

lib/logitech_receiver/device.py

@@ -40,6 +40,7 @@ from . import settings
 from . import settings_templates
 from .common import Alert
 from .common import Battery
+from .common import BatteryStatus
 from .common import _read_usb_product_string
 from .hidpp10_constants import NotificationFlag
 from .hidpp20_constants import SupportedFeature
@@ -609,6 +610,14 @@ class Device:
                     self.read_battery()  # battery information may have changed so try to read it now
             elif was_active and self.receiver and not isinstance(self.receiver, CenturionReceiver):
                 hidpp10.set_configuration_pending_flags(self.receiver, 0xFF)
+            if not active and self.receiver and self.battery_info is not None and self.battery_info.level is not None:
+                self.battery_info = Battery(
+                    self.battery_info.level,
+                    self.battery_info.next_level,
+                    BatteryStatus.OFFLINE,
+                    self.battery_info.voltage,
+                    self.battery_info.light_level,
+                )
         if logger.isEnabledFor(logging.DEBUG):
             logger.debug("device %d changed: active=%s %s", self.number, self._active, self.battery_info)
         if self.status_callback is not None:

tests/logitech_receiver/test_common.py

@@ -306,6 +306,7 @@ def test_kw_exception():
             "Battery: low (slow recharge)",
         ),
         (common.BatteryStatus.DISCHARGING, None, True, False, ""),
+        (common.BatteryStatus.OFFLINE, None, True, False, ""),
     ],
 )
 def test_battery(status, expected_level, expected_ok, expected_charging, expected_string):
@@ -326,3 +327,13 @@ def test_battery_2():
     assert battery.ok()
     assert not battery.charging()
     assert battery.to_str() == "Battery: 50% (discharging)"
+
+
+def test_battery_offline():
+    battery = common.Battery(58, None, common.BatteryStatus.OFFLINE, None)
+
+    assert battery.status == common.BatteryStatus.OFFLINE
+    assert battery.level == 58
+    assert battery.ok()
+    assert not battery.charging()
+    assert battery.to_str() == "Battery: 58% (offline)"