feat(payments-next): add passkey sign-in and Accounts-style buttons to checkout
Because - Accounts redesigned to add passkey sign-in; checkout should offer same options This commit - Adds AuthOptionButton to payments-ui, matching the Accounts BoxButton - Shows Sign in with passkey above Google and Apple on the checkout start page - Routes passkey through the Accounts login page, as passkeys only work there - Gates the passkey button behind FEATURE_FLAG_PASSKEY_SIGNIN (off by default) - Adds start page tests for button order, the flag, and signIn arguments Closes PAY-3842