fix(builder): Image block placeholder no longer overridden by explicit src=""

BlocksPanel dropped a new Image block with an explicit `src=""` prop, which
overrides ImageBlock's `src = PLACEHOLDER_SRC` default parameter (defaults
only apply when a prop is undefined, not when it's an empty string). Craft
then persisted `src:''`, and the canvas rendered a broken-image icon instead
of the placeholder.

- ImageBlock render now falls back to PLACEHOLDER_SRC whenever src is falsy
  (belt-and-braces: also recovers any legacy saved src:'' state).
- BlocksPanel no longer passes src="" when dropping a new Image block, so
  the craft default applies.
- ImageStylePanel now restores the placeholder (instead of blanking to '')
  when the URL field is cleared.

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

craft/src/components/media/ImageBlock.render.test.tsx

@@ -0,0 +1,62 @@
+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';
+
+/* ImageBlock only needs useNode from @craftjs/core. Mock it following the
+   DOM-harness pattern in src/components/basic/Footer.editguard.test.tsx (no
+   @testing-library/react in this repo) so we can render the real component
+   tree and inspect the emitted <img src> without a real <Editor>. */
+vi.mock('@craftjs/core', () => ({
+  useNode: (collect?: (node: any) => any) => {
+    const node = { events: { selected: false } };
+    return {
+      connectors: { connect: (el: any) => el, drag: (el: any) => el },
+      actions: { setProp: vi.fn() },
+      ...(collect ? collect(node) : {}),
+    };
+  },
+}));
+
+import { ImageBlock } from './ImageBlock';
+
+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(() => {
+  vi.clearAllMocks();
+});
+
+describe('ImageBlock render falls back to the placeholder for an explicit empty src (Bug 1)', () => {
+  test('src="" (explicit, overrides the default parameter) still renders a non-empty placeholder src', () => {
+    render(<ImageBlock src="" alt="Image" />);
+    const img = container.querySelector('img')!;
+    expect(img.getAttribute('src')).not.toBe('');
+    expect(img.getAttribute('src')).toMatch(/^data:image\/svg\+xml/);
+    container.remove();
+  });
+
+  test('src=undefined (default parameter path) still renders the placeholder (unchanged behavior)', () => {
+    render(<ImageBlock alt="Image" />);
+    const img = container.querySelector('img')!;
+    expect(img.getAttribute('src')).not.toBe('');
+    expect(img.getAttribute('src')).toMatch(/^data:image\/svg\+xml/);
+    container.remove();
+  });
+
+  test('a real src is rendered unchanged', () => {
+    render(<ImageBlock src="https://example.com/photo.jpg" alt="A photo" />);
+    const img = container.querySelector('img')!;
+    expect(img.getAttribute('src')).toBe('https://example.com/photo.jpg');
+    container.remove();
+  });
+});

craft/src/components/media/ImageBlock.tsx

@@ -3,7 +3,7 @@ import { useNode, UserComponent } from '@craftjs/core';
 import { cssPropsToString } from '../../utils/style-helpers';
 import { escapeAttr, safeUrl } from '../../utils/escape';
 
-const PLACEHOLDER_SRC = "data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='400' height='300'%3E%3Cdefs%3E%3ClinearGradient id='bg' x1='0' y1='0' x2='0' y2='1'%3E%3Cstop offset='0%25' stop-color='%23f1f5f9'/%3E%3Cstop offset='100%25' stop-color='%23e2e8f0'/%3E%3C/linearGradient%3E%3C/defs%3E%3Crect fill='url(%23bg)' width='400' height='300' rx='12'/%3E%3Crect x='2' y='2' width='396' height='296' rx='10' fill='none' stroke='%23cbd5e1' stroke-width='2' stroke-dasharray='8 4'/%3E%3Cg transform='translate(200,110)'%3E%3Crect x='-28' y='-28' width='56' height='56' rx='12' fill='%23cbd5e1' opacity='0.5'/%3E%3Cpath d='M-12 8 L-4 -2 L2 4 L8 -6 L16 8Z' fill='%2394a3b8'/%3E%3Ccircle cx='-6' cy='-10' r='5' fill='%2394a3b8'/%3E%3C/g%3E%3Ctext x='200' y='160' text-anchor='middle' fill='%2364748b' font-family='Inter,sans-serif' font-size='15' font-weight='500'%3EDrop image here%3C/text%3E%3Ctext x='200' y='182' text-anchor='middle' fill='%2394a3b8' font-family='Inter,sans-serif' font-size='12'%3Eor click to upload%3C/text%3E%3C/svg%3E";
+export const PLACEHOLDER_SRC = "data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='400' height='300'%3E%3Cdefs%3E%3ClinearGradient id='bg' x1='0' y1='0' x2='0' y2='1'%3E%3Cstop offset='0%25' stop-color='%23f1f5f9'/%3E%3Cstop offset='100%25' stop-color='%23e2e8f0'/%3E%3C/linearGradient%3E%3C/defs%3E%3Crect fill='url(%23bg)' width='400' height='300' rx='12'/%3E%3Crect x='2' y='2' width='396' height='296' rx='10' fill='none' stroke='%23cbd5e1' stroke-width='2' stroke-dasharray='8 4'/%3E%3Cg transform='translate(200,110)'%3E%3Crect x='-28' y='-28' width='56' height='56' rx='12' fill='%23cbd5e1' opacity='0.5'/%3E%3Cpath d='M-12 8 L-4 -2 L2 4 L8 -6 L16 8Z' fill='%2394a3b8'/%3E%3Ccircle cx='-6' cy='-10' r='5' fill='%2394a3b8'/%3E%3C/g%3E%3Ctext x='200' y='160' text-anchor='middle' fill='%2364748b' font-family='Inter,sans-serif' font-size='15' font-weight='500'%3EDrop image here%3C/text%3E%3Ctext x='200' y='182' text-anchor='middle' fill='%2394a3b8' font-family='Inter,sans-serif' font-size='12'%3Eor click to upload%3C/text%3E%3C/svg%3E";
 
 interface ImageBlockProps {
   src?: string;
@@ -66,7 +66,7 @@ export const ImageBlock: UserComponent<ImageBlockProps> = ({
         imgRef.current = ref;
         if (ref) connect(drag(ref));
       }}
-      src={src}
+      src={src || PLACEHOLDER_SRC}
       alt={alt || 'Image'}
       onDrop={handleDrop}
       onDragOver={handleDragOver}