June to October Improvements: Spaces, Sell, Worksheets and Invites

A batch of improvements across spaces, Sell, worksheets, attachments and invites:

• Duplicated spaces keep their setup: each app instance is copied with its own name and settings, and module prerequisite locks and ordering carry over.
• One-time offer coupons: the coupon is applied before checkout, so the platform fee is calculated on the discounted price.
• Coupon redemption caps: the cap sits on the promotion code, so a forever discount keeps applying on later invoices after the code reaches its redemption limit.
• Checkout feedback: when a checkout doesn’t complete, the buyer is shown a message.
• Long worksheet answers are readable in the response overview.
• PDF attachments: downloads skip the image optimizer, so PDFs open from their download link.
• Invites: blank email rows are ignored, and an invalid address shows a validation message.