Update dusky_sliders.py

[ADIOR-enigma]

Implementation

• Adds external monitor brightness control via ddcutil (DDC/CI)
• Automatic detection of DDC-capable displays
• New slider integrated into existing GTK4 UI (Only enabled when ddcutil is available and displays are detected)
• Uses LatestValueExecutor for non-blocking control
• Grace period handling to prevent UI desync

Details

• Uses ddcutil getvcp/setvcp (VCP code 0x10)
• Cached display detection to avoid repeated slow scans
• Separate refresh grace timing due to slower I2C operations
• Follows existing architecture