fix: fix styling issues for saved methods in dc

[jakubjasinsky]

Summary

Fix styling issues for saved payment methods in Dynamic Checkout — adjusts the payment methods manager modal and billing address select inputs.

Changes

• Payment methods manager modal: Add 10px gap between saved payment methods, remove border from list, align delete button to the right, remove hover states from delete and cog buttons
• Billing address select placeholder: Align placeholder color (#696F79) to match other input placeholders — uses a --placeholder modifier class that's removed on selection
• Button cleanup: Remove min-width, border-radius, and transition from cog/delete buttons; use justify-content: flex-end for right-alignment