feat: add multi-battery monitoring

- Report battery data for individual devices
- Select a representative battery for legacy fields and alerts
- Average named battery data independently
- Display multiple batteries in system charts
- Add cross-platform coverage and transport tests

agent/battery/battery_stub.go

@@ -8,6 +8,6 @@ func HasReadableBattery() bool {
 	return false
 }
 
-func GetBatteryStats() (uint8, uint8, error) {
-	return 0, 0, errors.ErrUnsupported
+func GetBatteryStats() ([]Battery, error) {
+	return nil, errors.ErrUnsupported
 }