chore: adjust UI of express payment methods in dc

[jakubjasinsky]

Summary

Unify the visual appearance of express checkout payment methods (Apple Pay, Google Pay, saved cards, saved APMs) with consistent sizing, spacing, and styling.

Changes

• Unify gap between wallet buttons and saved payment methods to 10px (was 20px/8px)
• Set all express checkout buttons to consistent 48px height
• Switch Apple Pay button to white-outline style, Google Pay to white with custom black border
• Add black borders to saved cards/APMs, matching wallet button styling
• Remove hover background changes on Google Pay, saved cards, and saved APMs
• Darken loading spinner in saved payment method buttons
• Increase border-radius of regular payment methods section to 12px
• Add margin-bottom to express checkout header to preserve spacing after gap reduction

Impact

• Visual changes only — no logic or API changes
• Apple Pay button now renders as white with outline (was black) — requires Safari to verify