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/features/project-editor/models/project-editor.model.ts

@@ -55,4 +55,6 @@ export const DEFAULT_EDITOR_HEADER_CONFIG: Required<HeaderConfig> = {
   showWishlist: true,
   showCompare: true,
   showRegion: true,
+  sticky: true,
+  layout: 'default',
 };