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/schemas/canvas.py

@@ -84,3 +84,8 @@ class CanvasStateResponse(BaseModel):
     edges: list[EdgeResponse]
     viewport: dict[str, Any]
     custom_style: dict[str, Any] | None = None
+    # True once this design's canvas has ever been persisted (a CanvasState row
+    # exists). Lets the frontend tell a brand-new user (show demo) apart from one
+    # who intentionally cleared their canvas (keep it empty). False also for a
+    # missing/uninitialized design.
+    initialized: bool = False