Fix book card button to render when url is set but amazonUrl is not; v1.1.38

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

src/App.tsx

@@ -1787,8 +1787,8 @@ function BooksSectionSection({ content }: { content: SiteContent }) {
               <div className="book-card-meta">
                 <span className="book-card-title">{book.label}</span>
                 {book.description && <p className="book-card-description">{book.description}</p>}
-                {book.amazonUrl && (
-                  <a href={book.amazonUrl} target="_blank" rel="noopener noreferrer" className="btn-primary book-card-btn">
+                {(book.amazonUrl || book.url) && (
+                  <a href={book.amazonUrl || book.url} target="_blank" rel="noopener noreferrer" className="btn-primary book-card-btn">
                     {book.amazonLabel || 'View on Amazon'}
                   </a>
                 )}