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,
     )
 
 

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

backend/tests/test_canvas.py

@@ -22,6 +22,23 @@ async def test_load_canvas_empty(client: AsyncClient, headers: dict):
     assert data["viewport"] == {"x": 0, "y": 0, "zoom": 1}
 
 
+async def test_load_canvas_uninitialized_reports_initialized_false(client: AsyncClient, headers: dict):
+    # A never-saved canvas has no CanvasState row → initialized False. The frontend
+    # uses this to show the demo canvas only to genuinely new users.
+    data = (await client.get("/api/v1/canvas", headers=headers)).json()
+    assert data["initialized"] is False
+
+
+async def test_load_canvas_initialized_true_after_save(client: AsyncClient, headers: dict):
+    # Saving an EMPTY canvas still creates a CanvasState row, so a subsequently
+    # loaded empty canvas is reported initialized — the user cleared it on purpose
+    # and must not get the demo re-seeded.
+    await client.post("/api/v1/canvas/save", json={"nodes": [], "edges": [], "viewport": {}}, headers=headers)
+    data = (await client.get("/api/v1/canvas", headers=headers)).json()
+    assert data["nodes"] == []
+    assert data["initialized"] is True
+
+
 async def test_load_canvas_requires_auth(client: AsyncClient):
     res = await client.get("/api/v1/canvas")
     assert res.status_code == 401