Campaign discount codes now apply when subscribing through the setup wizard

The Subi setup wizard's pricing step shows the active discount-campaign price on plan cards, but subscribing from that step did not carry the discount code through to the actual Shopify charge, so the approval page showed and charged the full plan price instead of the discounted one merchants had just seen.

Subscribing from the wizard now sends both the campaign discount code and the billing period along with the plan selection, and the charge is created with that discount applied, the same way it already worked when changing plans from outside the wizard. The price a merchant sees on the plan card during setup is now the price they are actually charged.

This only affects new merchants subscribing to a plan during onboarding while a discount campaign is active; merchants who set up their plan outside of an active campaign, or through the regular plan-change flow, were not affected.