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/__tests__/canvasLoadDecision.test.ts

@@ -0,0 +1,26 @@
+import { describe, it, expect } from 'vitest'
+import { decideCanvasLoad, isNewUserCanvas } from '../canvasLoadDecision'
+
+describe('decideCanvasLoad', () => {
+  it('renders real nodes when the canvas has any', () => {
+    expect(decideCanvasLoad(true, true)).toBe('real')
+    // hasNodes wins even if the initialized flag is somehow false
+    expect(decideCanvasLoad(true, false)).toBe('real')
+  })
+
+  it('keeps an initialized-but-empty canvas empty (user cleared it)', () => {
+    expect(decideCanvasLoad(false, true)).toBe('empty')
+  })
+
+  it('shows the demo only for a brand-new, uninitialized canvas', () => {
+    expect(decideCanvasLoad(false, false)).toBe('demo')
+  })
+})
+
+describe('isNewUserCanvas', () => {
+  it('is true only for the demo mode', () => {
+    expect(isNewUserCanvas('demo')).toBe(true)
+    expect(isNewUserCanvas('empty')).toBe(false)
+    expect(isNewUserCanvas('real')).toBe(false)
+  })
+})