Simplify calibration model, add pixel preview, and improve UI

- Replace segment-based calibration with core parameters (leds_top/right/bottom/left);
  segments are now derived at runtime via lookup tables
- Fix clockwise/counterclockwise edge traversal order for all 8 start_position/layout
  combinations (e.g. bottom_left+clockwise now correctly goes up-left first)
- Add pixel layout preview overlay with color-coded edges, LED index labels,
  direction arrows, and start position marker
- Move "Add New Device" form into a modal dialog triggered by "+" button
- Add display index selector to device settings modal
- Migrate from requirements.txt to pyproject.toml for dependency management
- Update Dockerfile and docs to use `pip install .`

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

server/src/wled_controller/api/routes.py

@@ -309,7 +309,6 @@ async def update_device(
             device_id=device_id,
             name=update_data.name,
             url=update_data.url,
-            led_count=update_data.led_count,
             enabled=update_data.enabled,
         )