fix: include items array in purchase dataLayer (#8339)

Include GA4-style `items` payload for subscription purchase events so
GTM receives the full item array.

So, to align with GA4's ecommerce standards, we changed it to an items
array:
https://developers.google.com/analytics/devguides/collection/ga4/item-scoped-ecommerce

src/platform/cloud/subscription/composables/useSubscription.test.ts

@@ -237,10 +237,14 @@ describe('useSubscription', () => {
         event: 'purchase',
         transaction_id: 'sub_123',
         currency: 'USD',
-        item_id: 'monthly_creator',
-        item_variant: 'monthly',
-        item_category: 'subscription',
-        quantity: 1
+        items: [
+          {
+            item_id: 'monthly_creator',
+            item_variant: 'monthly',
+            item_category: 'subscription',
+            quantity: 1
+          }
+        ]
       })
       expect(localStorage.getItem('pending_subscription_purchase')).toBeNull()
     })