Topic: Missing User Feedback for Unactivated Coupon Promotions

Description:
When a customer enters a coupon code linked to a promotion during checkout, it is technically validated.

If the promotion cannot be activated due to the current contents of the shopping cart (e.g., due to product category or discount restrictions), the customer receives no feedback on the front end.

From the customer's perspective, nothing happens – even though the code would be valid.

Current Behavior (Default):
- Coupon is checked in the background.
- No error message or information is displayed if conditions are not met.
- The customer does not know whether the code is invalid or if there are simply no matching items in the shopping cart at the moment.

Expected Behavior / Improvement Suggestion:
- Display of feedback such as:
"The coupon has been accepted, but there are currently no matching items in the shopping cart."
- Alternatively: A generic message stating that the coupon will be activated as soon as matching items are added to the shopping cart.

Rationale:
- Increases transparency and customer trust.
- Avoids frustration caused by seemingly "non-functional" coupons.
- Improves user experience without impacting existing promotion logic.

Suggested implementation:
- Add an informational message to the frontend (e.g., in the sw-promotion-code-form or via AddCodeError/AddCodeSuccess handling).
- Alternatively: A backend event hook that checks promotions and provides a flag or message object.