feat(site-builder): add hero, services, about and cta sections to the library

Token-based sections (21 total across four families) with tests covering
insert, export (no stray hex, alt on images, relative links) and tree shape.

Refs #52

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

@@ -53,7 +53,9 @@ describe('SectionLibraryModal', () => {
     h.act(() => openSectionLibrary());
     const chip = (l: string) => $$('.section-lib-chip').find((c) => c.textContent === l)!;
     h.act(() => chip('Hero').click());
-    expect($$('.section-lib-card').map((c) => c.dataset.sectionId)).toEqual(['hero-centered']);
+    const heroIds = $$('.section-lib-card').map((c) => c.dataset.sectionId);
+    expect(heroIds).toContain('hero-centered');
+    expect(heroIds.every((id) => id!.startsWith('hero-'))).toBe(true);
     h.act(() => chip('All').click());
     type($('.section-lib-search') as HTMLInputElement, 'form');
     expect($$('.section-lib-card').map((c) => c.dataset.sectionId)).toContain('contact-form');