Manually creating a prepaid subscription now charges the full term

Creating a subscription by hand on a prepaid plan now charges the full prepaid term, instead of pricing it as a single delivery.

What was wrong. A by-hand prepaid subscription built from the Create subscription page was priced for one delivery rather than the whole term it was supposed to cover, and it didn't carry a bundle, box, or volume discount that should have applied from the moment it was created.

What's fixed. The create page now previews the exact amount that will be charged before you save, built from a live backend calculation rather than a client-side guess; if that preview fails to load, Save stays disabled rather than letting you save an unverified price. The page also now refuses to combine products that are on different plans in the same manual subscription, and shows the first-billing result the backend actually applies on the success screen after you save.

This only affects subscriptions created by hand through the Create subscription page; subscriptions created through storefront checkout were not affected by this bug.