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/AdminPage.tsx

@@ -3905,7 +3905,7 @@ async function handleAssetUpload(event: ChangeEvent<HTMLInputElement>) {
                       <input id={`book-amazon-label-${book.id}`} type="text" value={book.amazonLabel ?? ''} placeholder="View on Amazon" onChange={e => updateLink(book.id, 'amazonLabel', e.target.value)} />
                     </div>
                     <div className="admin-field">
-                      <label htmlFor={`book-url-${book.id}`}>Link URL <span className="admin-field-optional">(optional fallback)</span></label>
+                      <label htmlFor={`book-url-${book.id}`}>Link URL <span className="admin-field-optional">(used if no Amazon URL is set)</span></label>
                       <input id={`book-url-${book.id}`} type="url" value={book.url ?? ''} placeholder="https://..." onChange={e => updateLink(book.id, 'url', e.target.value)} />
                     </div>
                   </div>