Add "Add to cart" button to product list page

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This PR adds "Add to cart" functionality directly to the product list page (product.php), allowing users to add items to their cart without having to navigate to the individual product detail pages.

Changes Made

• Added "Add to cart" button to each product card in the product list
• Placed in the existing empty block2-txt-child2 div that was previously unused
• Reuses existing cart logic from carthandler.php with identical URL parameters
• Maintains consistent styling using the same CSS classes as the detail page button

Implementation Details

The button uses the same onclick handler pattern as the existing implementation in details.php:

<button onclick="location.href='carthandler.php?cart_id=<?php echo $final['id'] ?>&cart_name=<?php echo $final['name'] ?>&cart_price=<?php echo $final['price'] ?>'" 
        class="flex-c-m stext-101 cl0 size-101 bg1 bor1 hov-btn1 p-lr-15 trans-04">
    Add to cart
</button>

User Experience Improvement

• Users can now add products to cart directly from the product listing
• No need to visit individual product pages for simple cart additions
• Maintains existing "More Details" functionality for users who want product information
• Consistent styling and behavior with existing cart functionality

Technical Notes

• Only 3 lines of code added to accomplish the requirement
• No changes needed to existing carthandler.php logic
• Maintains all existing functionality and styling
• PHP sessions and cart management already properly configured

Fixes #28.

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