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/schemas.py

@@ -90,15 +90,6 @@ class ProcessingSettings(BaseModel):
     )
 
 
-class CalibrationSegment(BaseModel):
-    """Calibration segment for LED mapping."""
-
-    edge: Literal["top", "right", "bottom", "left"] = Field(description="Screen edge")
-    led_start: int = Field(description="Starting LED index", ge=0)
-    led_count: int = Field(description="Number of LEDs on this edge", gt=0)
-    reverse: bool = Field(default=False, description="Reverse LED order on this edge")
-
-
 class Calibration(BaseModel):
     """Calibration configuration for pixel-to-LED mapping."""
 
@@ -108,18 +99,17 @@ class Calibration(BaseModel):
     )
     start_position: Literal["top_left", "top_right", "bottom_left", "bottom_right"] = Field(
         default="bottom_left",
-        description="Position of LED index 0"
+        description="Starting corner of the LED strip"
     )
     offset: int = Field(
         default=0,
         ge=0,
         description="Number of LEDs from physical LED 0 to start corner (along strip direction)"
     )
-    segments: List[CalibrationSegment] = Field(
-        description="LED segments for each screen edge",
-        min_length=1,
-        max_length=4
-    )
+    leds_top: int = Field(default=0, ge=0, description="Number of LEDs on the top edge")
+    leds_right: int = Field(default=0, ge=0, description="Number of LEDs on the right edge")
+    leds_bottom: int = Field(default=0, ge=0, description="Number of LEDs on the bottom edge")
+    leds_left: int = Field(default=0, ge=0, description="Number of LEDs on the left edge")
 
 
 class CalibrationTestModeRequest(BaseModel):