fix(preview): add navigation block placeholder as first block

The header is the first block on the real site — added it to the
preview BLOCKS array so it shows as placeholder before Hero.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/web/app/pages/preview/PreviewClient.tsx

@@ -36,6 +36,12 @@ const BLOCKS: Array<{
   ready: boolean;
   component?: React.ReactNode;
 }> = [
+  {
+    id: "navigation",
+    name: "Шапка / Навигация",
+    href: "/components/navigation",
+    ready: false,
+  },
   {
     id: "hero",
     name: "Hero-баннер",