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

scripts/seed_demo.py

@@ -146,6 +146,11 @@ def process(tick, mode="comfort", away=False, setpoint=21.0):
         "indoor": round(indoor, 2),
         "outdoor": round(outdoor, 2),
         "humidity": round(humidity, 1),
+        # Not a reading: what the panel on the wall should wear. The dashboard
+        # binds its `theme` setting to this, which is what "switch the panel at
+        # sunset" looks like here — a flow publishing, not a rule in the
+        # dashboard.
+        "panel_theme": "dark" if hour < 7.0 or hour >= 20.0 else "light",
     }
 '''
 
@@ -331,6 +336,8 @@ HOUSE_NODES = [
             # downstream of it is paced by this rather than by the tick.
             {"name": "outdoor", "dtype": "float", "interval": 60},
             {"name": "humidity", "dtype": "float"},
+            # Twice a day at most, so it is paced rather than sent every tick.
+            {"name": "panel_theme", "dtype": "str", "interval": 60},
         ],
     },
     {
@@ -1214,6 +1221,18 @@ def seed_dashboard(api: Api) -> None:
             "columns": COLUMNS,
             "canvas_width": CANVAS[0],
             "canvas_height": CANVAS[1],
+            # The settings channel: the dashboard itself as a receiver. Costs
+            # no tile, which is why it fits a panel with all 17 rows spoken
+            # for. `system` is the fallback until the flow first says
+            # otherwise; `locked` is left unset, which is the static default
+            # and keeps every control on the demo usable.
+            "settings": {
+                "theme": {
+                    "value": "system",
+                    "message": msg(HOUSE, "panel_theme"),
+                    "dtype": "str",
+                }
+            },
             "pages": PAGES,
         },
     )