feat(builder): send + restore site head code in save/load

Extend the save payload with head_code + design so the backend can
inject SiteDesign.headCode into published pages, and restore design
tokens on load() so the editor reflects the last-saved state.

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

craft/src/hooks/useWhpApi.save.test.ts

@@ -1,6 +1,7 @@
 import { describe, test, expect } from 'vitest';
 import { buildSavePayload } from './useWhpApi';
 import { PageData } from '../types';
+import { DEFAULT_SITE_DESIGN } from '../state/SiteDesignContext';
 
 const pageA: PageData = { id: 'home', name: 'Home', slug: 'index', craftState: 'STORED_HOME' };
 const pageB: PageData = { id: 'page_2', name: 'About', slug: 'about', craftState: 'STORED_ABOUT' };
@@ -19,6 +20,8 @@ describe('buildSavePayload', () => {
       activePageId: '__header__',
       isEditingHeader: true,
       isEditingFooter: false,
+      headCode: DEFAULT_SITE_DESIGN.headCode,
+      design: DEFAULT_SITE_DESIGN,
     });
 
     // The fresh live canvas must be reflected in header_craft_state, not the stale stored one.
@@ -52,6 +55,8 @@ describe('buildSavePayload', () => {
       activePageId: '__footer__',
       isEditingHeader: false,
       isEditingFooter: true,
+      headCode: DEFAULT_SITE_DESIGN.headCode,
+      design: DEFAULT_SITE_DESIGN,
     });
 
     expect(payload.footer_craft_state).toBe('LIVE_FOOTER');
@@ -75,6 +80,8 @@ describe('buildSavePayload', () => {
       activePageId: 'home',
       isEditingHeader: false,
       isEditingFooter: false,
+      headCode: DEFAULT_SITE_DESIGN.headCode,
+      design: DEFAULT_SITE_DESIGN,
     });
 
     // Live canvas goes to the active page and top-level slots.
@@ -105,6 +112,8 @@ describe('buildSavePayload', () => {
       activePageId: 'home',
       isEditingHeader: false,
       isEditingFooter: false,
+      headCode: DEFAULT_SITE_DESIGN.headCode,
+      design: DEFAULT_SITE_DESIGN,
     });
 
     // The live edit must land in pages_craft_state[0] (index.html slot),
@@ -116,4 +125,26 @@ describe('buildSavePayload', () => {
     // The other page is untouched.
     expect(payload.pages_craft_state.find((p) => p.id === 'p2')?.craftState).toBe('STORED_ABOUT_2');
   });
+
+  test('head code + design tokens are included in the save payload', () => {
+    const design = { ...DEFAULT_SITE_DESIGN, headCode: '<meta name="x">' };
+
+    const payload = buildSavePayload({
+      siteId: 1,
+      siteName: 'Test Site',
+      liveCraftState: 'LIVE_PAGE_HOME',
+      pages: [pageA, pageB],
+      headerPage,
+      footerPage,
+      activePageId: 'home',
+      isEditingHeader: false,
+      isEditingFooter: false,
+      headCode: '<meta name="x">',
+      design,
+    });
+
+    expect(payload.head_code).toBe('<meta name="x">');
+    expect(payload.design).toEqual(design);
+    expect(payload.design.headCode).toBe('<meta name="x">');
+  });
 });