Add admin quiz attempts viewer

- /admin/quizzes: list all quizzes with question and attempt counts
- /admin/quizzes/[quizId]: view all student attempts with answers per question
- Add "Тесты" link to admin sidebar navigation

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

src/components/admin/admin-nav.tsx

@@ -9,6 +9,7 @@ const links = [
   { href: "/admin/categories", label: "Категории" },
   { href: "/admin/users", label: "Пользователи" },
   { href: "/curator/homework", label: "ДЗ на проверку" },
+  { href: "/admin/quizzes", label: "Тесты" },
   { href: "/admin/comments", label: "Комментарии" },
   { href: "/admin/import-export", label: "Импорт / Экспорт" },
   { href: "/admin/settings", label: "Настройки" },