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/fluksio/flow/panels.py

@@ -85,11 +85,15 @@ def find(panel_id: str) -> PanelDef | None:
 
 
 def messages_for(panel_id: str, store: DashboardStore) -> set[str]:
-    """Every message the widgets of this panel's dashboards read or write.
+    """Every message this panel's dashboards read or write.
 
     What a screen is entitled to see, as its own dashboards define it. Read
     from the published documents, since that is what a panel draws, and empty
     for a panel that is gone — which is the same answer as "nothing".
+
+    A dashboard's own bound settings count, not only its widgets': the theme a
+    panel is driven to is a message no tile on it draws, and a wall panel
+    refused its own theme message is the one surface the setting exists for.
     """
     panel = find(panel_id)
     if panel is None:
@@ -100,6 +104,7 @@ def messages_for(panel_id: str, store: DashboardStore) -> set[str]:
             defn = store.read(dashboard)
         except DashboardNotFound:
             continue
+        names.update(defn.setting_messages)
         for widget in defn.widgets:
             names.update(widget.messages)
             if widget.target: