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/client/schemas.gen.ts

@@ -3138,7 +3138,7 @@ export const WidgetDefSchema = {
         },
         type: {
             type: 'string',
-            enum: ['stat', 'gauge', 'chart', 'markdown', 'agenda', 'notification', 'bar', 'icon', 'forecast', 'clock', 'button', 'switch', 'slider', 'input', 'dropdown'],
+            enum: ['stat', 'gauge', 'chart', 'markdown', 'agenda', 'notification', 'bar', 'icon', 'forecast', 'clock', 'button', 'switch', 'slider', 'input', 'dropdown', 'color'],
             title: 'Type'
         },
         title: {
@@ -3173,7 +3173,20 @@ message. One with \`\`source: "query"\`\` asks instead, and its config is
 \`\`{source, request, request_dtype: "record", message, dtype: "series",
 refresh_s, range_s}\`\`: it publishes \`\`{range_s, interval_s}\`\` to
 \`\`request\`\` exactly as a slider publishes a value, and draws the \`\`series\`\`
-a flow answers with on \`\`message\`\`.`
+a flow answers with on \`\`message\`\`.
+
+A colour widget picks what it publishes with \`\`format\`\`, because fixtures
+differ and a change node per tile is not the answer:
+
+- \`\`hsv\`\` (the default) — \`\`[h, s, v]\`\`, hue 0-360 degrees, saturation and
+  value 0-100 percent. What the Node-RED installation this ports feeds its
+  3CH/4CH DMX encoders, which divide by 360 and by 100.
+- \`\`rgb\`\` — \`\`[r, g, b]\`\`, each 0-255. The conventional range; the
+  reference's own encoders produce it after converting.
+- \`\`hex\`\` — \`\`"#rrggbb"\`\`, lowercase. Conventional likewise.
+
+The first two are a \`\`list\`\` message, the third a \`\`str\`\`, which is what
+\`\`COLOR_DTYPES\`\` records and the check below holds a binding to.`
 } as const;
 
 export const WorkerInfoSchema = {