* fix(sales): allow gift card credit for returns + surface errors
- Add incrementGiftcardValue() to Giftcard model for crediting cards
- In Sale::save_value(), use incrementGiftcardValue() for returns
with negative gift card payments (credits the card)
- Add error_message rendering to register view so failed completions
show the error instead of silently reloading
Fixes#4779
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Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>