[web] Render address form above payment element on Stripe checkout (#36209)

* [web] render address form above payment element on Stripe checkout

Changing country across Stripe regions (US/CA vs rest) remounts the Elements
tree and clears any entered card details. Rendering the address form (which
contains the country selector) first lets users settle the country before
entering payment details.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* [web] gate checkout address-first order behind checkout-address-order split test

Per PR review: the reorder needs testing, so it is now behind the
checkout-address-order split test (default keeps the current payment-first
order). With the enabled variant the heading becomes "Payment details", since
"Select a payment method" no longer sits above the payment element.

Part of #36429

* Rename `checkout-address-order` to `checkout-address-first`

GitOrigin-RevId: 9c861b1013a284b10757fc416bcdc5610790d582

services/web/frontend/extracted-translations.json

@@ -1457,6 +1457,7 @@
   "pause_subscription": "",
   "pause_subscription_for": "",
   "pay_now": "",
+  "payment_details": "",
   "payment_error_3ds_failed": "",
   "payment_error_generic": "",
   "payment_error_intermittent_error": "",

services/web/locales/en.json

@@ -1894,6 +1894,7 @@
   "pause_subscription": "Pause subscription",
   "pause_subscription_for": "Pause subscription for",
   "pay_now": "Pay now",
+  "payment_details": "Payment details",
   "payment_error_3ds_failed": "We couldn’t complete your payment because authentication wasn’t successful. Please try again or choose a different payment method. If the problem continues please <0>contact us</0>.",
   "payment_error_generic": "Sorry, something went wrong. Please try again. If the problem continues please <0>contact us</0>.",
   "payment_error_intermittent_error": "We were unable to process your payment. Please try again later or <0>contact us</0> for assistance.",