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}}\"?",

src/locales/fa.json

@@ -2917,7 +2917,7 @@
       "metaDesc": "توضیحات متا",
       "active": "فعال",
       "inactive": "غیرفعال",
-      "purchases": "خرید",
+      "purchaseCount": "خرید",
       "copyUrl": "کپی URL",
       "urlCopied": "URL کپی شد",
       "deleteConfirm": "حذف صفحه فرود «{{title}}»؟",

src/locales/ru.json

@@ -3743,7 +3743,7 @@
       "metaDesc": "Meta Description",
       "active": "Активен",
       "inactive": "Неактивен",
-      "purchases": "покупок",
+      "purchaseCount": "покупок",
       "copyUrl": "Скопировать URL",
       "urlCopied": "URL скопирован",
       "deleteConfirm": "Удалить лендинг «{{title}}»?",

src/locales/zh.json

@@ -2916,7 +2916,7 @@
       "metaDesc": "Meta描述",
       "active": "已激活",
       "inactive": "未激活",
-      "purchases": "次购买",
+      "purchaseCount": "次购买",
       "copyUrl": "复制URL",
       "urlCopied": "URL已复制",
       "deleteConfirm": "删除落地页「{{title}}」？",

src/pages/AdminLandings.tsx

@@ -180,7 +180,7 @@ function SortableLandingCard({
               </div>
               <div className="text-sm text-dark-400">
                 <span>
-                  {landing.purchase_stats.total} {t('admin.landings.purchases')}
+                  {landing.purchase_stats.total} {t('admin.landings.purchaseCount')}
                 </span>
               </div>
             </div>