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/routes/color_strip_sources.py

@@ -51,6 +51,7 @@ def _css_to_response(source) -> ColorStripSourceResponse:
         gamma=getattr(source, "gamma", None),
         smoothing=getattr(source, "smoothing", None),
         interpolation_mode=getattr(source, "interpolation_mode", None),
+        led_count=getattr(source, "led_count", 0),
         calibration=calibration,
         description=source.description,
         created_at=source.created_at,
@@ -93,6 +94,7 @@ async def create_color_strip_source(
             gamma=data.gamma,
             smoothing=data.smoothing,
             interpolation_mode=data.interpolation_mode,
+            led_count=data.led_count,
             calibration=calibration,
             description=data.description,
         )
@@ -143,6 +145,7 @@ async def update_color_strip_source(
             gamma=data.gamma,
             smoothing=data.smoothing,
             interpolation_mode=data.interpolation_mode,
+            led_count=data.led_count,
             calibration=calibration,
             description=data.description,
         )