[web] Remove SubscriptionFormatters, format subscription dates in the frontend (#36409)

* [web] Remove SubscriptionFormatters, format subscription dates in the frontend

Replace the server-formatted nextPaymentDueAt/nextPaymentDueDate/
trialEndsAtFormatted fields with the raw periodEnd date and format in
the React dashboard (same strings as before, English + UTC). Also
consolidate hand-rolled trial checks onto isInFreeTrial /
SubscriptionHelper.isInTrial.

* [web] Remove the now-unused dateformat dependency

* [web] Address review: payment-dates unit test, UTC dates in acceptance test

Add a dedicated unit test asserting the exact formatted strings from
formatPaymentDate/formatPaymentDateTime, and use explicit UTC timestamps
in the SubscriptionDashboard acceptance fixtures instead of
local-timezone Date constructors.

GitOrigin-RevId: 16ac4f1098b14ef9f1478d22cf39c1357ead7dea

services/web/frontend/js/features/subscription/util/is-in-free-trial.ts

@@ -1,4 +1,6 @@
-export default function isInFreeTrial(trialEndsAt?: string | null) {
+export default function isInFreeTrial(
+  trialEndsAt?: string | null
+): trialEndsAt is string {
   if (!trialEndsAt) return false
 
   const endDate = new Date(trialEndsAt)

services/web/frontend/js/features/subscription/util/payment-dates.ts

@@ -0,0 +1,15 @@
+import { formatTime } from '@/features/utils/format-date'
+
+export function formatPaymentDate(date: string | null | undefined) {
+  if (!date) {
+    return null
+  }
+  return formatTime(date, 'MMMM Do, YYYY', true)
+}
+
+export function formatPaymentDateTime(date: string | null | undefined) {
+  if (!date) {
+    return null
+  }
+  return formatTime(date, 'MMMM Do, YYYY h:mm A [UTC]', true)
+}