[Merged] Add "Set as Default Payment Method" Option to the Default UI

[Merged into We want an existing card to be automatically marked as the default during a new subscription purchase]

🔍Problem Statement
Currently, when a customer adds a new payment method through the Default UI, it is not automatically set as the default payment method. As a result, customers must navigate to their saved payment methods and manually change the default before they can remove the old card. This creates unnecessary friction and can lead to failed renewals if the old default payment method remains in place.

💡User Story
As a customer updating my payment method, I want the option to set my newly added payment method as the default during the add payment method flow, so that I don't have to update it separately afterward.

🎯Definition of Done

✔ Add a "Set as default payment method" checkbox to the Default UI payment method modal.

✔When selected, the newly added payment method is immediately set as the customer's default payment method after it is successfully added.