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/js/features/devices.js

@@ -100,9 +100,7 @@ export function createDeviceCard(device) {
                 <button class="btn btn-icon btn-secondary" onclick="showSettings('${device.id}')" title="${t('device.button.settings')}">
                     ⚙️
                 </button>
-                <button class="btn btn-icon btn-secondary" onclick="showCalibration('${device.id}')" title="${t('device.button.calibrate')}">
-                    📐
-                </button>
+
             </div>
         </div>
     `;