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/shop-product/_partial/product-custom-fields.php

@@ -4,7 +4,7 @@
       <?= $custom_field['name'];?>:
     </div>
     <div class="_input">
-      <input type="text" class="form-control" name="custom_field[<?= $custom_field['id_additional_field'];?>]" field_name="<?= $custom_field['name'];?>" value="">
+      <input type="text" class="form-control" name="custom_field[<?= $custom_field['id_additional_field'];?>]" field_name="<?= $custom_field['name'];?>" value="" <? if ( !empty( $custom_field['is_required'] ) ): ?>required<? endif; ?>>
     </div>
   </div>
 <? endforeach; endif;?>