fix(site-builder): re-read saved sections when the library opens

Refs #52

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

craft/src/panels/sections/SectionLibraryModal.test.tsx

@@ -100,4 +100,18 @@ describe('SectionLibraryModal', () => {
     h.act(() => { ($('.section-lib-delete') as HTMLElement).click(); });
     expect(extrasApi.get<any[]>('customSections', [])).toHaveLength(0);
   });
+  test('sections loaded silently after mount (replaceAll) appear in My sections on open', () => {
+    mount();
+    // Mirrors useWhpApi: the project load calls replaceAll, which does not re-render.
+    h.act(() => {
+      extrasApi.replaceAll({
+        customSections: [{
+          id: 'custom-loaded', name: 'Loaded hero', family: 'custom', createdAt: 1,
+          tree: { type: { resolvedName: 'Section' }, props: {} },
+        }],
+      });
+    });
+    h.act(() => openSectionLibrary());
+    expect($$('.section-lib-card').map((c) => c.dataset.sectionId)).toContain('custom-loaded');
+  });
 });