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/static/locales/en.json

@@ -552,6 +552,7 @@
   "color_strip.interpolation.dominant": "Dominant",
   "color_strip.smoothing": "Smoothing:",
   "color_strip.smoothing.hint": "Temporal blending between frames (0=none, 1=full). Reduces flicker.",
+  "color_strip.color_corrections": "Color Corrections",
   "color_strip.brightness": "Brightness:",
   "color_strip.brightness.hint": "Output brightness multiplier (0=off, 1=unchanged, 2=double). Applied after color extraction.",
   "color_strip.saturation": "Saturation:",
@@ -561,6 +562,8 @@
   "color_strip.test_device": "Test on Device:",
   "color_strip.test_device.hint": "Select a device to send test pixels to when clicking edge toggles",
   "color_strip.leds": "LED count",
+  "color_strip.led_count": "LED Count:",
+  "color_strip.led_count.hint": "Total number of LEDs on the physical strip. Set to 0 to use the sum from calibration. Useful when the strip has LEDs behind the TV that are not mapped to screen edges — those LEDs will be sent black.",
   "color_strip.created": "Color strip source created",
   "color_strip.updated": "Color strip source updated",
   "color_strip.deleted": "Color strip source deleted",