Let the slider controls take their colours from the slider in 1.11.3

Dots, arrows, counter and button background are style properties of the
slider ("Slider-Steuerung"), written as custom properties onto the slider and
onto the control bar the script places after it, so the script-free markers
and the scripted controls share the same colours.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

public/assets/design-panel.js

@@ -18,6 +18,7 @@ export const GROUPS = {
   image: 'Bild',
   layout: 'Layout',
   effect: 'Wirkung',
+  slider: 'Slider-Steuerung',
 };
 
 // dvh before vh: on a phone 100vh is taller than the screen, 100dvh is what people mean.