Show empty-canvas hint on a page with no components yet

.empty-canvas-hint existed in editor.css but was never rendered
anywhere. Wire it up in Canvas.tsx: an EmptyCanvasHint component reads
Craft's ROOT node via useEditor and shows the hint once ROOT exists
with zero children, hiding again the instant something is dropped in
or while a drag is in progress. It's absolutely positioned over the
Frame with pointer-events: none so it never intercepts clicks/drops
meant for the underlying (empty) canvas -- scoped to regular page
editing only, not the header/footer editing mode.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

craft/src/editor/EmptyCanvasHint.test.tsx

@@ -0,0 +1,67 @@
+import { describe, test, expect, vi, beforeEach } from 'vitest';
+import React from 'react';
+import { createRoot, Root } from 'react-dom/client';
+import { act } from 'react-dom/test-utils';
+
+/* Same DOM-harness pattern as Footer.editguard.test.tsx: mock @craftjs/core's
+   useEditor so we can drive editor state without a real <Editor> tree. */
+let mockNodes: Record<string, { data: { nodes: string[] } }> = {};
+let mockDraggedSize = 0;
+
+vi.mock('@craftjs/core', () => ({
+  useEditor: (collect: (state: any) => any) =>
+    collect({
+      nodes: mockNodes,
+      events: { dragged: { size: mockDraggedSize } },
+    }),
+}));
+
+import { EmptyCanvasHint } from './Canvas';
+
+let container: HTMLDivElement;
+let root: Root;
+
+function render(ui: React.ReactElement) {
+  container = document.createElement('div');
+  document.body.appendChild(container);
+  act(() => {
+    root = createRoot(container);
+    root.render(ui);
+  });
+}
+
+beforeEach(() => {
+  mockNodes = {};
+  mockDraggedSize = 0;
+});
+
+describe('EmptyCanvasHint', () => {
+  test('renders nothing before ROOT has mounted (no root node yet)', () => {
+    render(<EmptyCanvasHint />);
+    expect(container.querySelector('.empty-canvas-hint')).toBeNull();
+    container.remove();
+  });
+
+  test('renders the hint once ROOT exists with zero children', () => {
+    mockNodes = { ROOT: { data: { nodes: [] } } };
+    render(<EmptyCanvasHint />);
+    expect(container.querySelector('.empty-canvas-hint')).not.toBeNull();
+    expect(container.textContent).toContain('Drag blocks from the left panel');
+    container.remove();
+  });
+
+  test('hides once the page has content', () => {
+    mockNodes = { ROOT: { data: { nodes: ['node-1'] } } };
+    render(<EmptyCanvasHint />);
+    expect(container.querySelector('.empty-canvas-hint')).toBeNull();
+    container.remove();
+  });
+
+  test('hides while a drag is in progress, even on an empty root', () => {
+    mockNodes = { ROOT: { data: { nodes: [] } } };
+    mockDraggedSize = 1;
+    render(<EmptyCanvasHint />);
+    expect(container.querySelector('.empty-canvas-hint')).toBeNull();
+    container.remove();
+  });
+});