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/fluksio/flow/dashboards.py

@@ -60,9 +60,17 @@ WidgetType = Literal[
     "slider",
     "input",
     "dropdown",
+    "color",
 ]
 
-INPUT_WIDGETS = {"button", "switch", "slider", "input", "dropdown"}
+INPUT_WIDGETS = {"button", "switch", "slider", "input", "dropdown", "color"}
+
+#: What a colour widget puts on the wire, by the format it was configured for.
+#: The default is what the Node-RED installation this ports already sends its
+#: DMX encoders — ``[h, s, v]``, hue in degrees and the other two in percent —
+#: and the two alternatives exist because fixtures differ. Mirrored in the
+#: client (``frontend/src/components/Dashboard/ColorWidget.tsx``).
+COLOR_DTYPES = {"hsv": "list", "rgb": "list", "hex": "str"}
 
 #: What a widget may be pointed at, by payload type. A switch that reads a
 #: float has nothing to show and nothing safe to send, so the pairing belongs
@@ -80,6 +88,9 @@ WIDGET_DTYPES: dict[str, set[str]] = {
     "notification": {"record"},
     "bar": {"float", "int"},
     "forecast": {"list"},
+    # Either shape a colour can travel as; which of the two this widget means
+    # is its ``format``, checked against ``COLOR_DTYPES`` below.
+    "color": {"list", "str"},
     # An icon maps weather strings, bool hints and numbers alike, and a clock
     # binds nothing at all, so neither has a row to be held to.
 }
@@ -107,6 +118,19 @@ class WidgetDef(BaseModel):
     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 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.
     """
 
     id: str
@@ -217,6 +241,19 @@ class WidgetDef(BaseModel):
         if isinstance(inner, list) and len(inner) > BAR_SEGMENTS:
             raise ValueError(f"a bar nests at most {BAR_SEGMENTS} readings")
 
+        if self.type == "color":
+            # The row above allows both shapes a colour travels as; the format
+            # is what decides which of them this widget actually sends. An
+            # unknown one is read as the default, exactly as the client does.
+            fmt = str(self.config.get("format") or "hsv")
+            want = COLOR_DTYPES.get(fmt, "list")
+            bound = str(self.config.get("dtype") or "")
+            if bound and bound != want:
+                raise ValueError(
+                    f"a colour widget sending {fmt} needs a '{want}' message, "
+                    f"not a '{bound}'"
+                )
+
         allowed = WIDGET_DTYPES.get(self.type)
         if not allowed:
             return self
@@ -558,6 +595,7 @@ def default_dashboard(name: str) -> DashboardDef:
 
 __all__ = [
     "BAR_SEGMENTS",
+    "COLOR_DTYPES",
     "DASHBOARD_DIR",
     "HISTORY_CAP",
     "INPUT_WIDGETS",