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/i18n/translations.ts

@@ -428,6 +428,10 @@ export interface Translations {
     socialImage: string;
     gallery: string;
     addGalleryImage: string;
+    headerLayout: string;
+    headerLayoutDefault: string;
+    headerLayoutCentered: string;
+    headerSticky: string;
     marketplaceTitle: string;
     primaryColor: string;
     secondaryColor: string;
@@ -575,6 +579,8 @@ export interface Translations {
     faviconDesc: string;
     socialImageDesc: string;
     galleryDesc: string;
+    headerLayoutDesc: string;
+    headerStickyDesc: string;
     marketplaceTitleDesc: string;
     primaryColorDesc: string;
     secondaryColorDesc: string;