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/components/header/header.component.html

@@ -1,5 +1,5 @@
   <!-- platform VERSION - Redesigned 2026 -->
-  <header class="platform-header">
+  <header class="platform-header" [class.platform-header--static]="headerConfig().sticky === false" [class.platform-header--centered]="headerConfig().layout === 'centered'">
     <div class="platform-header-container">
       <!-- Logo -->
       @if (headerConfig().showLogo) {