Let the slider keep the drag the browser wanted to take

Dragging the brightness slider on the wall panel set the handle and
published nothing; tapping a point on the track worked. The control is a
native range input laid transparent over the drawn track, and only the
release publishes — but the input never said the drag was its own. On a
touch panel a sideways swipe is a pan, or a back-navigation, so the
browser took the pointer over mid-drag and ended it in `pointercancel`.
The value had followed the finger and was never sent.

`touch-action: none`, as the colour disk beside it has always had. The
release also answers `pointercancel` and `lostpointercapture` now, which
covers a mouse let go outside the input and leaves no way for a draft to
sit there unpublished.

Two checks: a drag across the track reaches the engine, and the input
still owns its gesture. The second fails against a build without the
CSS, which is what makes it worth having.

frontend/src/components/Dashboard/ui/core/controls.ts

@@ -203,6 +203,13 @@ export function useSliderDrag({
       onChange: (event: React.ChangeEvent<HTMLInputElement>) =>
         setDraft(Number(event.target.value)),
       onPointerUp: release,
+      // A drag can end without a `pointerup` on this element: the browser can
+      // claim the gesture and cancel the pointer, and a mouse released outside
+      // the input reports the release elsewhere. Either way the draft would sit
+      // there unpublished, showing a value nobody was ever sent. `release`
+      // ignores a second call, so overlapping ends are free.
+      onPointerCancel: release,
+      onLostPointerCapture: release,
       onKeyUp: release,
       onBlur: release,
     },

frontend/src/components/Dashboard/ui/core/core.css

@@ -250,6 +250,13 @@
   background: none;
   opacity: 0;
   cursor: pointer;
+  /* The drag is the control, so the browser may not read it as a gesture of
+     its own. Without this a sideways swipe on a touch panel is a pan — or a
+     back-navigation — and the browser takes the pointer over mid-drag, which
+     ends the drag in `pointercancel` rather than `pointerup`. The value
+     followed the finger and was never published. Same reason `.dui-disc`
+     has it. */
+  touch-action: none;
 }
 
 .dui-slider[data-orientation="vertical"] .dui-slider-input {