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/en.ts

@@ -430,6 +430,10 @@ export const en: Translations = {
     socialImage: 'Social Share Image',
     gallery: 'Gallery',
     addGalleryImage: 'Add image',
+    headerLayout: 'Header Layout',
+    headerLayoutDefault: 'Default',
+    headerLayoutCentered: 'Centered',
+    headerSticky: 'Sticky Header',
     marketplaceTitle: 'Marketplace Title',
     primaryColor: 'Primary Color',
     secondaryColor: 'Secondary Color',
@@ -577,6 +581,8 @@ export const en: Translations = {
     faviconDesc: 'Browser tab icon image URL. Should be a small square image.',
     socialImageDesc: 'Default image shown when the marketplace is shared on social media (Open Graph).',
     galleryDesc: 'Extra brand images available for future use across the storefront.',
+    headerLayoutDesc: 'How the header content is arranged: default (logo left, nav spread) or centered.',
+    headerStickyDesc: 'Keep the header pinned to the top of the viewport while scrolling.',
     marketplaceTitleDesc: 'The page title used by search engines and browser tabs.',
     primaryColorDesc: 'The main brand color, used on primary buttons and highlights. Must be a valid hex color.',
     secondaryColorDesc: 'A supporting brand color used for secondary UI elements. Must be a valid hex color.',