refactor: move navigation page from /components to /blocks section

- Moved /components/navigation → /blocks/navigation
- Updated Sidebar: navigation is now first item in "Блоки"
- Updated PreviewClient block path
- Updated seed block path
- Updated SPRINTS.md

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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

@@ -49,8 +49,8 @@ const BLOCKS: BlockDef[] = [
   {
     id: "navigation",
     name: "Шапка / Навигация",
-    href: "/components/navigation",
-    path: "/components/navigation",
+    href: "/blocks/navigation",
+    path: "/blocks/navigation",
     defaultReady: true,
     component: <NavigationBlock />,
   },