web: the mixer's card can be dragged off the colour it was read at

The card hangs on the point the eyedropper read, which is exactly where the
user wants to watch the band move — so it covers the patch it is editing. Its
body now takes a drag: the offset is a fraction of the photo, which is the
layer the card lives in, so a zoom keeps it where it was put and a fresh pick
drops it back on its own point. The knobs and the close button keep the pointer
to themselves, and the anchor cannot leave the photo, so the card is always
half in reach of a drag back.

docker/frontend/src/styles/app.css

@@ -357,6 +357,10 @@ input[type="range"] { width: 100%; accent-color: var(--accent); }
 .hsl-panel {
   position: absolute;
   pointer-events: auto;
+  /* Dragged by its body to a clear patch of photo; the knobs and the close
+     button keep the pointer to themselves. */
+  cursor: grab;
+  touch-action: none;
   width: 248px;
   padding: 8px;
   display: flex;