Add backlight control for Litra Beam LX

[Copilot]

Adds programmatic control of the Litra Beam LX backlight via driver functions and CLI commands.

Driver API

Three new functions in driver.ts:

• backlightOn(device) - Sends HID command 0x11 0xff 0x0a 0x4b 0x01
• backlightOff(device) - Sends HID command 0x11 0xff 0x0a 0x4b 0x00
• isBacklightOn(device) - Queries backlight state via 0x11 0xff 0x0a 0x3b

All functions validate device type and throw for non-Beam LX devices.

CLI Commands

• litra-backlight-on - Turn on backlight
• litra-backlight-off - Turn off backlight

Both support --serial-number for multi-device setups.

Usage

import { findDevice, backlightOn, backlightOff, isBacklightOn } from 'litra';

const device = findDevice();

if (device && device.type === 'litra_beam_lx') {
  backlightOn(device);
  console.log(isBacklightOn(device)); // true
  
  backlightOff(device);
  console.log(isBacklightOn(device)); // false
}

litra-backlight-on --serial-number ABC123
litra-backlight-off --serial-number ABC123

Device Support

Litra Beam LX only. Functions throw "Backlight control is only supported on Litra Beam LX devices" for Litra Glow and Litra Beam.

Original prompt

The current driver implementation does not provide functionality to control the backlight of Logitech Litra devices. There is a need to introduce new commands such as back-on, back-off, and potentially others to allow users to manage the backlight state programmatically, similar to the existing power and brightness controls.

Issue created by Visual Studio Copilot

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