Staff may charge a customer's saved card at the till but cannot add or save
one. A customer picker (reusing the admin user-search pattern, excluding
admin/guest/affiliate) loads the customer's cards via
GET /admin/users/{id}/payment-methods; the 'Saved card' payment option is
hidden outright when the customer has no currently-valid cards, computed
client-side with the Square convention (valid through the end of
exp_month/exp_year). The saved-card charge sends payment_method saved_card
plus user_id/user_saved_card_id with no card_token or verification_token, and
the per-line idempotency keys also key on the selected card so switching cards
yields fresh keys. No Square Dashboard hint, no new-card form, no save
checkbox — the till can never persist a card.