feat(backoffice): add page preview carousel on book detail page

Shows 5 pages at a time in a full-width grid with prev/next navigation.
Pages are fetched via the existing proxy route with webp format.

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

apps/backoffice/app/books/[id]/page.tsx

@@ -1,4 +1,5 @@
 import { fetchLibraries, getBookCoverUrl, BookDto, apiFetch } from "../../../lib/api";
+import { BookPreview } from "../../components/BookPreview";
 import Image from "next/image";
 import Link from "next/link";
 import { notFound } from "next/navigation";
@@ -157,6 +158,12 @@ export default async function BookDetailPage({
           </div>
         </div>
       </div>
+
+      {book.page_count && book.page_count > 0 && (
+        <div className="mt-8">
+          <BookPreview bookId={book.id} pageCount={book.page_count} />
+        </div>
+      )}
     </>
   );
 }