Add display brightness and power control

- New display service with DDC/CI brightness and power control via screen_brightness_control and monitorcontrol
- New /api/display/* endpoints (monitors, brightness, power)
- Display tab in WebUI with per-monitor brightness sliders and power toggle
- EDID resolution parsing to distinguish same-name monitors
- Throttled brightness slider (50ms) matching volume control pattern

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pyproject.toml

@@ -40,6 +40,8 @@ windows = [
     "pywin32>=306",
     "comtypes>=1.2.0",
     "pycaw>=20230407",
+    "screen-brightness-control>=0.20.0",
+    "monitorcontrol>=3.0.0",
 ]
 dev = [
     "pytest>=7.0",