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>

README.md

@@ -42,7 +42,7 @@ This project consists of two components:
 
 2. **Install dependencies**
    ```bash
-   pip install -r requirements.txt
+   pip install .
    ```
 
 3. **Run the server**
@@ -150,7 +150,7 @@ wled-screen-controller/
 │   ├── src/wled_controller/ # Main application code
 │   ├── tests/               # Unit and integration tests
 │   ├── config/              # Configuration files
-│   └── requirements.txt     # Python dependencies
+│   └── pyproject.toml       # Python dependencies & project config
 ├── homeassistant/           # Home Assistant integration
 │   └── custom_components/
 └── docs/                    # Documentation