feat(canvas): distinguish new user from cleared canvas; stop demo on backend error

Cleared canvas re-showed the demo, and backend errors silently fell back to
demo — hiding real outages and forcing users to wipe demo nodes before bulk
edits. Now:
- backend load reports `initialized` (CanvasState row exists = ever saved)
- decideCanvasLoad() picks real | empty | demo; empty is kept empty
- backend down/error shows an error banner + toast, never the demo
- data-new-user flag reserved as the Getting Started walkthrough hook

ha-relevant: yes

backend/app/api/routes/canvas.py

@@ -37,6 +37,9 @@ async def load_canvas(
         edges=[EdgeResponse.model_validate(e) for e in edges],
         viewport=viewport,
         custom_style=state.custom_style if state else None,
+        # A CanvasState row exists only after a save (or explicit design create),
+        # so its presence marks an intentional canvas vs. a never-touched one.
+        initialized=state is not None,
     )