Rework the dashboard into two looks over one behaviour

A dashboard is a wall panel somebody hangs in their own hallway, so it
now wears what they choose: a look, and a palette of their own colours.

Two complete component sets live under `Dashboard/ui/` — `glass`
(translucent panes over a slowly moving ground) and `material` (Material
3 tonal cards) — behind one prop contract. Every control's state,
keyboard and `aria-` live in `ui/core` and are shared, so the two sets
are the same dashboard drawn twice rather than two products: a set only
decides what a control looks like while doing it.

Four settings join the channel, each drivable by a flow like any other:
`look`, `palette`, `background` and `touch`. A palette is an ordered list
of hex colours — background, surface, primary, accent, text, then more
chart colours — pasted from a coolors.co link or typed, written onto the
canvas as the token variables everything already reads. Trailing roles
are derived, so three colours are a whole dashboard, and derived text is
held to AA rather than trusted (`theme.check.ts` measures it). A palette
also decides light or dark, since its first colour is the ground.

Widgets are measured against their own tile with container queries rather
than against the viewport, animate through `motion`, and can be drawn
without their title. The three reworks:

- a bar draws a row per reading, up to eight, each in the dashboard's own
  data colours and each able to carry its own scale — replacing readings
  nested in one fill, which could only ever share one colour and stop at
  three. Documents written the old way are read as rows.
- a chart's range picker moved to a column down its right-hand edge, which
  gives the plot back a whole row of a short tile.
- the colour wheel became a disc: hue is the angle and saturation the
  distance from the middle, so a colour is one gesture rather than three,
  with brightness on a slider beside it.

`index.css` and `lib/motion.ts` are untouched — the dashboard overrides
token *values* on its canvas, never the blocks the two repos share.

backend/tests/flow/test_dashboards.py

@@ -284,6 +284,37 @@ def test_a_bar_is_drawn_on_both_readings_it_nests():
     assert widget.messages == ["a.in", "a.pv"]
 
 
+def test_a_bar_reads_its_rows():
+    widget = WidgetDef(
+        id="b",
+        type="bar",
+        config={
+            "rows": [
+                {"message": "a.load", "dtype": "float", "label": "House"},
+                {"message": "a.pv", "dtype": "int"},
+            ],
+            "min": 0,
+            "max": 9,
+        },
+    )
+
+    assert widget.messages == ["a.load", "a.pv"]
+
+    with pytest.raises(ValueError):
+        WidgetDef(
+            id="b",
+            type="bar",
+            config={"rows": [{"message": "a.on", "dtype": "bool"}]},
+        )
+
+    with pytest.raises(ValueError):
+        WidgetDef(
+            id="b",
+            type="bar",
+            config={"rows": [{"message": f"a.m{n}"} for n in range(9)]},
+        )
+
+
 def test_a_clock_reads_nothing_and_publishes_nothing():
     widget = WidgetDef(id="c", type="clock", config={"format": "24h"})