Add the dashboard settings channel, wired for theme and lock

A dashboard could only ever receive as a set of tiles. This adds the dashboard
itself as a receiver: `settings` maps a name to a value plus an optional
binding. Unbound, the setting is simply its value — a wall panel that is always
dark costs no flow. Bound, a flow drives it live and the value is the fallback.

Two settings are wired: `theme` (system/light/dark) and `locked` (read-only).
There is no schedule field on purpose — a node publishing to the bound message
on a cron is what a schedule is here, which is the point of a channel.

- `messages_for()` now walks a dashboard's bound settings as well as its
  widgets' bindings. Without this a paired screen is refused its own theme
  message, on the one surface the setting exists for; it bounds the socket too.
- `locked` is gated in `usePublish`, so every control inherits it, and each
  control also draws itself disabled — a dead button reads as broken otherwise.
  The panel surface says Read-only in the corner.
- The theme is a class on the dashboard's own surface, never the root: inside
  the app shell it must not flip the chrome. `.light` gains the tokens `.dark`
  already had (mirrored in the index repo) so both directions work on a subtree.
- Settings bindings are type-checked from the document alone, the rule widget
  bindings follow, and mirrored on the server.
- A bound setting is drawn on the flow canvas as a dashboard-level endpoint.
- The demo's house flow now publishes `home.panel_theme`, which the demo
  dashboard's theme binds to: the panel goes dark after sunset, at no tile cost.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018tULRZJUkZsw7rMJ3h4xvu

backend/tests/flow/test_dashboards.py

@@ -8,6 +8,7 @@ from fluksio.flow.dashboards import (
     DashboardStore,
     PageDef,
     SectionDef,
+    SettingDef,
     WidgetDef,
     default_dashboard,
 )
@@ -353,3 +354,62 @@ def test_a_dashboard_is_cut_into_a_sane_number_of_columns():
     for columns in (0, 49):
         with pytest.raises(ValueError):
             DashboardDef(name="house", columns=columns)
+
+
+def test_a_setting_refuses_a_message_it_cannot_carry():
+    """The same rule a widget binding is held to, from the document alone."""
+    DashboardDef(
+        name="house",
+        settings={"theme": SettingDef(value="dark", message="a.b", dtype="str")},
+    )
+    DashboardDef(
+        name="house",
+        settings={"locked": SettingDef(value=False, message="a.b", dtype="bool")},
+    )
+
+    with pytest.raises(ValueError):
+        DashboardDef(
+            name="house",
+            settings={"theme": SettingDef(value="dark", message="a.b", dtype="float")},
+        )
+    with pytest.raises(ValueError):
+        DashboardDef(
+            name="house",
+            settings={"locked": SettingDef(value=False, message="a.b", dtype="str")},
+        )
+
+
+def test_an_unbound_setting_is_just_its_value():
+    """The static case, which is what makes a dark panel cost no flow.
+
+    No message means nothing to type-check and nothing for a panel to be
+    entitled to — and a name this build does not know is left alone rather
+    than refused, so an older installation reads a newer document.
+    """
+    defn = DashboardDef(
+        name="house",
+        settings={
+            "theme": SettingDef(value="dark"),
+            "someday": SettingDef(value=7, message="a.b", dtype="int"),
+        },
+    )
+
+    assert defn.settings["theme"].value == "dark"
+    assert defn.setting_messages == ["a.b"]
+
+
+def test_a_bound_setting_is_drawn_on_the_canvas(store: DashboardStore):
+    """A dashboard consuming a message is an endpoint like a tile is."""
+    store.write(
+        DashboardDef(
+            name="house",
+            settings={"theme": SettingDef(value="system", message="home.theme")},
+        )
+    )
+
+    (binding,) = store.bindings_for("home")
+    assert binding["widget"] == "settings.theme"
+    assert binding["type"] == "setting"
+    assert binding["requires"] == ["home.theme"]
+    assert not binding["provides"]
+    assert store.bindings_for("other") == []