When coupons reduced cart total to $0, the checkout page displayed no
button to finalize the order. This occurred because the Stripe button
only appears with active_card, and crypto buttons only appear when
cart.requires_payment is True. Users without saved payment methods
were unable to complete free orders.
Added dedicated "Confirm Free Checkout" button that displays when
cart.requires_payment is False, allowing users to finalize free orders.
Enhanced test coverage to verify button appears in checkout page HTML.
