Create CardSelection.svelte reusable component encapsulating the standard
saved-card list + 'Use a new card' + CardInput pattern with blur-based
validation (Luhn, expiry, CVC) — identical to the tip flows and account page.
Refactor UserPaymentModal (Make a Payment submodal) to use CardSelection:
- Removed its bespoke 'Use a different card' expand/collapse UI and inline
validation derivations (parseExpiryParts, isValidLuhn, touched state)
- Bound selectedCardId + new card fields to the component
- payButtonDisabled now driven by component's onValidityChange callback
- Removed now-unused CardInput import, SvelteDate import, formatCardExpiry
Fix account 'Buy a Gift Card' bug: 'Use a new card' click did nothing because
the auto-select effect immediately re-set buySelectedCard back to the default
card. Added buyShowNewCard flag so the effect only auto-selects on initial load;
reset after successful new-card purchase so the next purchase re-defaults.
popertots
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