feat(site-builder): per-page SEO meta + favicon + design-token CSS-var wiring + published-output a11y/perf

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

craft/src/state/SiteDesignContext.tsx

@@ -24,6 +24,9 @@ export interface SiteDesign {
 
   // Site-wide custom code
   headCode: string;
+
+  /** Site-wide favicon URL (one per site, not per-page). Empty string = none. */
+  favicon: string;
 }
 
 export interface SiteDesignContextValue {
@@ -52,6 +55,7 @@ export const DEFAULT_SITE_DESIGN: SiteDesign = {
   buttonRadius: '8px',
   navStyle: 'light',
   headCode: '',
+  favicon: '',
 };
 
 const SiteDesignContext = createContext<SiteDesignContextValue>({