fix(slider): enable layer rendering for opacity animations

Add layer.enabled: true to NColorSlider and NSlider widgets to fix
opacity animation issues. This ensures proper compositing during
slider animations by rendering to an offscreen layer.

Fixes visual glitches where slider opacity transitions were not
rendering correctly.

Widgets/NColorSlider.qml

@@ -174,4 +174,6 @@ Slider {
       }
     }
   }
+
+  layer.enabled: true
 }

Widgets/NSlider.qml

@@ -141,4 +141,6 @@ Slider {
       }
     }
   }
+
+  layer.enabled: true
 }