Enhance product management and coupon features

- Added styling for input group add-ons and additional fields in SCSS.
- Updated shop coupon view to include a new column for usage count.
- Display coupon code and amount in order details if applicable.
- Improved product edit template to handle custom fields with required validation.
- Modified product save logic to include custom field requirements.
- Enhanced decimal normalization function for better input handling.
- Implemented checkbox normalization for form submissions.
- Updated custom fields in product templates to reflect required status.
- Fixed URL for fetching changelog updates.

templates_user/shop-product/product.php

@@ -537,7 +537,7 @@
       }
 
       // dodatkowe pola muszą być uzupełnione
-      $( '.custom-field textarea' ).each( function( index, element )
+      $( '.custom-field textarea[required]' ).each( function( index, element )
       {
         if ( $.trim( $( element ).val() ) == '' )
         {