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/api/routes/test_panels.py

@@ -313,6 +313,86 @@ def test_a_panel_speaks_only_its_own_widgets_messages(
     )
 
 
+def test_a_panel_may_read_its_dashboards_own_settings(
+    client: TestClient, superuser_token_headers: dict[str, str]
+) -> None:
+    """A bound setting is no widget's binding, and a panel still needs it.
+
+    The allowlist is a walk of what the dashboards name, so the walk has to
+    reach the dashboard itself: a theme driven over a message is the one thing
+    a screen on a wall cannot be told any other way, and a panel refused that
+    message would leave the feature silently doing nothing on the only surface
+    it exists for.
+    """
+    from fluksio.api.routes.flows import panel_scope
+
+    _dashboard_with(
+        client, superuser_token_headers, "panel_themed", [PANEL_WIDGETS[-1]]
+    )
+    saved = client.get(
+        f"{DASHBOARDS}/panel_themed?draft=true", headers=superuser_token_headers
+    ).json()
+    saved["settings"] = {
+        "theme": {"value": "dark", "message": "demo.panel_theme", "dtype": "str"},
+        # Bound to nothing: a static setting entitles a panel to nothing.
+        "locked": {"value": False},
+    }
+    written = client.put(
+        f"{DASHBOARDS}/panel_themed", headers=superuser_token_headers, json=saved
+    )
+    assert written.status_code == 200, written.text
+    published = client.post(
+        f"{DASHBOARDS}/panel_themed/publish",
+        headers=superuser_token_headers,
+        json={"version": written.json()["version"]},
+    )
+    assert published.status_code == 200, published.text
+
+    _panels(
+        client,
+        superuser_token_headers,
+        {"panels": [{"id": "hall", "dashboards": ["panel_themed"]}]},
+    )
+    panel_headers = _pair(client, superuser_token_headers, "hall")
+    messages = f"{settings.API_V1_STR}/messages"
+
+    # Not 403: the gate lets it through, and the engine then answers for a
+    # message no flow in this suite declares.
+    assert (
+        client.get(
+            f"{messages}/demo.panel_theme/history", headers=panel_headers
+        ).status_code
+        == 200
+    )
+    # And the socket is bounded by the same walk, so the value actually lands.
+    token = panel_headers["Authorization"][7:]
+    assert panel_scope(token, client.app) == {
+        "demo.panel_theme",
+        "demo.temperature",
+    }
+
+    # A setting still buys nothing beyond itself.
+    assert (
+        client.get(
+            f"{messages}/demo.unrelated/history", headers=panel_headers
+        ).status_code
+        == 403
+    )
+
+
+def test_a_dashboard_setting_bound_to_the_wrong_type_is_refused(
+    client: TestClient, superuser_token_headers: dict[str, str]
+) -> None:
+    """The same answer the server gives a mis-wired widget."""
+    saved = _dashboard(client, superuser_token_headers, "panel_mistyped")
+    saved["settings"] = {"theme": {"value": "dark", "message": "a.b", "dtype": "float"}}
+
+    refused = client.put(
+        f"{DASHBOARDS}/panel_mistyped", headers=superuser_token_headers, json=saved
+    )
+    assert refused.status_code == 422, refused.text
+
+
 def test_unpairing_a_screen_leaves_its_panel_standing(
     client: TestClient, superuser_token_headers: dict[str, str]
 ) -> None: