Fix add-to-cart on home page: wrong class and data attribute

Button used add-to-cart/data-id instead of add-to-cart-btn/data-product-id.
Added inline onclick to match shop and product pages.

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

index.php

@@ -117,7 +117,7 @@ require_once __DIR__ . '/includes/header.php';
                                     <span class="original"><?= formatCurrency($product['price']) ?></span>
                                 <?php endif; ?>
                             </div>
-                            <button class="btn btn-primary btn-block add-to-cart" data-id="<?= $product['product_id'] ?>">
+                            <button class="btn btn-primary btn-block add-to-cart-btn" data-product-id="<?= $product['product_id'] ?>" onclick="addToCart('<?= $product['product_id'] ?>', 1)">
                                 <i class="fas fa-shopping-bag"></i> Add to Cart
                             </button>
                         </div>