feat(preview): add NavigationBlock component, show header in preview

NavigationBlock extracted from navigation/page.tsx into reusable component.
Preview now shows real header (ready: true) as first block — 3 of 8 ready.

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

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

@@ -1,6 +1,7 @@
 "use client";
 
 import { useState, useEffect } from "react";
+import { NavigationBlock } from "@/components/blocks/NavigationBlock";
 import { HeroBlock } from "@/components/blocks/HeroBlock";
 import { DoctorsBlock } from "@/components/blocks/DoctorsBlock";
 
@@ -40,7 +41,8 @@ const BLOCKS: Array<{
     id: "navigation",
     name: "Шапка / Навигация",
     href: "/components/navigation",
-    ready: false,
+    ready: true,
+    component: <NavigationBlock />,
   },
   {
     id: "hero",