feat(project-editor): header layout + sticky option

- HeaderConfig gains sticky (default true) and layout ('default'|'centered')
- header editor exposes layout select + sticky toggle
- runtime header component applies static/centered classes from config

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/shared/models/config/header-config.model.ts

@@ -1,3 +1,5 @@
+export type HeaderLayout = 'default' | 'centered';
+
 export interface HeaderConfig {
   showLogo?: boolean;
   showSearch?: boolean;
@@ -8,6 +10,8 @@ export interface HeaderConfig {
   showWishlist?: boolean;
   showCompare?: boolean;
   showRegion?: boolean;
+  sticky?: boolean;
+  layout?: HeaderLayout;
 }
 
 export const DEFAULT_HEADER_CONFIG: Required<HeaderConfig> = {
@@ -20,4 +24,6 @@ export const DEFAULT_HEADER_CONFIG: Required<HeaderConfig> = {
   showWishlist: true,
   showCompare: true,
   showRegion: true,
+  sticky: true,
+  layout: 'default',
 };