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/Common/DashboardMosaic.tsx

@@ -28,7 +28,14 @@ const PREVIEWS = 8
 
 /** Widgets that draw a shape, and widgets that are controls. The rest read out. */
 const GRAPHIC = new Set(["chart", "forecast", "bar", "gauge"])
-const INPUT = new Set(["button", "switch", "slider", "input", "dropdown"])
+const INPUT = new Set([
+  "button",
+  "switch",
+  "slider",
+  "input",
+  "dropdown",
+  "color",
+])
 
 const shade = (type: string) =>
   INPUT.has(type)