CSS: add led_count field; calibration dialog improvements; color corrections collapsible section

- Add explicit led_count to PictureColorStripSource (0 = auto from calibration)
- Stream pads with black or truncates to match led_count exactly
- Calibration dialog: show led_count input above visual editor in CSS mode
- Calibration dialog: pre-populate led_count with effective count (cal sum) when stored value is 0
- Calibration dialog: sync preview label live as led_count input changes
- CSS editor: group brightness/saturation/gamma into collapsible "Color Corrections" section

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

server/src/wled_controller/api/schemas/color_strip_sources.py

@@ -20,6 +20,7 @@ class ColorStripSourceCreate(BaseModel):
     gamma: float = Field(default=1.0, description="Gamma correction (1.0=none, <1=brighter, >1=darker mids)", ge=0.1, le=3.0)
     smoothing: float = Field(default=0.3, description="Temporal smoothing (0.0=none, 1.0=full)", ge=0.0, le=1.0)
     interpolation_mode: str = Field(default="average", description="LED color interpolation mode (average, median, dominant)")
+    led_count: int = Field(default=0, description="Total LED count (0 = auto from calibration)", ge=0)
     calibration: Optional[Calibration] = Field(None, description="LED calibration (position and count per edge)")
     description: Optional[str] = Field(None, description="Optional description", max_length=500)
 
@@ -35,6 +36,7 @@ class ColorStripSourceUpdate(BaseModel):
     gamma: Optional[float] = Field(None, description="Gamma correction (0.1-3.0)", ge=0.1, le=3.0)
     smoothing: Optional[float] = Field(None, description="Temporal smoothing (0.0-1.0)", ge=0.0, le=1.0)
     interpolation_mode: Optional[str] = Field(None, description="Interpolation mode (average, median, dominant)")
+    led_count: Optional[int] = Field(None, description="Total LED count (0 = auto from calibration)", ge=0)
     calibration: Optional[Calibration] = Field(None, description="LED calibration")
     description: Optional[str] = Field(None, description="Optional description", max_length=500)
 
@@ -52,6 +54,7 @@ class ColorStripSourceResponse(BaseModel):
     gamma: Optional[float] = Field(None, description="Gamma correction")
     smoothing: Optional[float] = Field(None, description="Temporal smoothing")
     interpolation_mode: Optional[str] = Field(None, description="Interpolation mode")
+    led_count: int = Field(0, description="Total LED count (0 = auto from calibration)")
     calibration: Optional[Calibration] = Field(None, description="LED calibration")
     description: Optional[str] = Field(None, description="Description")
     created_at: datetime = Field(description="Creation timestamp")