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.

backend/tests/flow/test_dashboards.py

@@ -303,6 +303,43 @@ def test_a_querying_chart_asks_with_a_record_and_draws_a_series():
         WidgetDef(id="c", type="chart", config=query_chart(request_dtype="json"))
 
 
+def test_a_colour_widget_publishes_the_shape_its_format_names():
+    """The format picks the payload, so the binding is held to that one."""
+    wheel = WidgetDef(
+        id="lamp",
+        type="color",
+        config={"target": "a.color", "dtype": "list", "format": "hsv"},
+    )
+
+    assert wheel.target == "a.color"
+    # It publishes rather than reads: nothing is drawn from a message.
+    assert wheel.messages == []
+    # Hex is the same widget speaking a string, and rgb is still a list.
+    WidgetDef(
+        id="l", type="color", config={"target": "a.c", "dtype": "str", "format": "hex"}
+    )
+    WidgetDef(
+        id="l", type="color", config={"target": "a.c", "dtype": "list", "format": "rgb"}
+    )
+    # Nothing recorded binds anything, as everywhere else.
+    WidgetDef(id="l", type="color", config={"target": "a.c"})
+
+    with pytest.raises(ValueError):
+        WidgetDef(
+            id="l",
+            type="color",
+            config={"target": "a.c", "dtype": "str", "format": "hsv"},
+        )
+    with pytest.raises(ValueError):
+        WidgetDef(
+            id="l",
+            type="color",
+            config={"target": "a.c", "dtype": "list", "format": "hex"},
+        )
+    with pytest.raises(ValueError):
+        WidgetDef(id="l", type="color", config={"target": "a.c", "dtype": "float"})
+
+
 def test_a_querying_chart_keeps_no_ring():
     """The answer carries its own past; a ring would store it twice."""
     widget = WidgetDef(