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/components/layout/Sidebar.tsx

@@ -30,12 +30,12 @@ const NAV: NavSection[] = [
       { label: "Кнопки", href: "/components/buttons" },
       { label: "Форм-контролы", href: "/components/forms" },
       { label: "Карточки", href: "/components/cards" },
-      { label: "Шапка / Навигация", href: "/components/navigation" },
     ],
   },
   {
     title: "Блоки",
     items: [
+      { label: "Шапка / Навигация", href: "/blocks/navigation" },
       { label: "Hero-баннер", href: "/blocks/hero" },
       { label: "Вводный текст", href: "/blocks/ceo" },
       { label: "Наши врачи", href: "/blocks/doctors" },