https://linear.app/unleash/issue/CTO-95/unleash-billing-page-for-enterprise-payg Adds support for PAYG in Unleash's billing page. Includes some refactoring, like splitting Pro and PAYG into different details components. We're now also relying on shared billing-related constants (see `BillingPlan.tsx`). This should make it much easier to change any of these values in the future. I already changed a few that were static / wrongly relying on instanceStatus.seats (we decided we're not doing that for now). ![image](https://github.com/user-attachments/assets/97a5a420-a4f6-4b6c-93d6-3fffddbacbc7)