fix: fallback to sysfs for battery percentage on HID devices

[jamerrq]

Closes #134

Problem

Devices like PS4/PS5 controllers report battery via the kernel HID driver
(/sys/class/power_supply/ps-controller-battery-XX:XX:XX:XX:XX:XX/capacity)
instead of the Bluetooth Battery Service profile. Since bluer only reads
from org.bluez.Battery1, these devices always show None for battery.

Solution

Added a read_battery_from_sysfs() fallback function that:

1. Converts the device MAC address to lowercase
2. Scans /sys/class/power_supply/ for entries containing the MAC address
3. Reads the capacity file to get the battery percentage
   This is used as a fallback via .or_else() only when bluer returns None.

Testing

Tested with:

• Sony DualShock 4 (PS4 controller) — reports battery level ok