Add a colour-wheel widget to the dashboard

A custom hue ring — a conic gradient, not a canvas — with saturation and
brightness sliders beside or under it depending on the tile's shape, sized
for a wall panel and reachable from a keyboard. It publishes [h, s, v] by
default, which is what the reference installation's DMX encoders read, and
`format` switches that to [r, g, b] or "#rrggbb".

`usePublish` moves to its own module so a widget in a file of its own can
reach it without importing `widgets.tsx` back.

frontend/src/components/Dashboard/dashboard.css

@@ -90,6 +90,51 @@
   box-shadow: inset 0 0 0 2px var(--primary);
 }
 
+/*
+ * The colour wheel, sized to whatever tile it was put on.
+ *
+ * `min(100cqw, 100cqh)` is what keeps one square inside a box of any shape
+ * without measuring anything in JS — the box is the query container, so the
+ * wheel reads its height as well as its width. The ring itself is a conic
+ * gradient set on the element, so nothing here repaints per frame.
+ */
+.widget-wheel-box {
+  container-type: size;
+}
+
+.widget-wheel {
+  width: min(100cqw, 100cqh);
+  aspect-ratio: 1;
+}
+
+/* The tile is its own query container, so the widget can be laid out by the
+   shape it was given rather than by the viewport — a wall panel's rows are
+   often wider than they are tall, and a wheel stacked above two sliders in one
+   of those is a dot. A container cannot answer a query about itself, which is
+   what the inner `-body` is for. */
+.widget-color {
+  container-type: size;
+}
+
+@container (min-aspect-ratio: 3 / 2) {
+  .widget-color-body {
+    flex-direction: row;
+    align-items: center;
+  }
+
+  /* Square by its height, taken from the tile: the wheel keeps whatever room
+     the row has and the components take the rest of the width. */
+  .widget-color-body > .widget-wheel-box {
+    flex: none;
+    width: 100cqh;
+  }
+
+  .widget-color-levels {
+    flex: 1;
+    min-width: 0;
+  }
+}
+
 /*
  * Motion. A value settling is a neutral state change; a selection indicator
  * moving is emphasized (Material). `<MotionConfig reducedMotion="user">` only