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/panels/left/BlocksPanel.tsx

@@ -123,7 +123,7 @@ const categories: CategoryDef[] = [
     label: 'Media',
     blocks: [
       { id: 'image', label: 'Image', icon: 'fa-image',
-        component: <ImageBlock src="" alt="Image" style={{ maxWidth: '100%', height: 'auto', display: 'block', borderRadius: '8px' }} /> },
+        component: <ImageBlock alt="Image" style={{ maxWidth: '100%', height: 'auto', display: 'block', borderRadius: '8px' }} /> },
       { id: 'video', label: 'Video', icon: 'fa-play-circle',
         component: <VideoBlock videoUrl="" isBackground={false} /> },
       { id: 'map-embed', label: 'Map', icon: 'fa-map-marker',