fix: rename duplicate 'purchases' i18n key to 'purchaseCount'

The admin.landings namespace had two 'purchases' keys at the same level:
a string for purchase count label and an object for purchases section.
JSON last-key-wins caused the string to be overwritten by the object,
producing "returned an object instead of string" error on landings list.

src/locales/en.json

@@ -3192,7 +3192,7 @@
       "metaDesc": "Meta Description",
       "active": "Active",
       "inactive": "Inactive",
-      "purchases": "purchases",
+      "purchaseCount": "purchases",
       "copyUrl": "Copy URL",
       "urlCopied": "URL copied",
       "deleteConfirm": "Delete landing \"{{title}}\"?",