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

frontend/src/utils/canvasLoadDecision.ts

@@ -0,0 +1,25 @@
+/**
+ * Decides what to render when a design's canvas loads.
+ *
+ * Three cases must stay distinct (issue: clearing a canvas re-showed the demo):
+ *  - `real`  → the canvas has saved nodes; render them.
+ *  - `empty` → the canvas is initialized (ever saved / explicitly created) but
+ *              has no nodes; the user intentionally cleared it — keep it empty.
+ *  - `demo`  → brand-new, never-initialized canvas; seed the demo so first-time
+ *              users see an example (and, later, the Getting Started walkthrough).
+ *
+ * Backend errors are handled by the caller, NOT here — a failed load must show an
+ * error and must never fall back to `demo` (that would hide the real failure).
+ */
+export type CanvasLoadMode = 'real' | 'empty' | 'demo'
+
+export function decideCanvasLoad(hasNodes: boolean, initialized: boolean): CanvasLoadMode {
+  if (hasNodes) return 'real'
+  if (initialized) return 'empty'
+  return 'demo'
+}
+
+/** A new user is one who lands on the demo canvas — the Getting Started hook. */
+export function isNewUserCanvas(mode: CanvasLoadMode): boolean {
+  return mode === 'demo'
+}