refactor: convert promocode stats modal to separate page

- Create AdminPromocodeStats.tsx with responsive mobile layout
- Add route /admin/promocodes/:id/stats
- Fix mobile layout in AdminPromocodes.tsx (badges and buttons no longer overlap)
- Remove PromocodeStatsModal component

src/locales/ru.json

@@ -2051,6 +2051,8 @@
         "save": "Сохранить"
       },
       "stats": {
+        "title": "Статистика промокода",
+        "notFound": "Промокод не найден",
         "totalUses": "Всего использований",
         "today": "Сегодня",
         "remaining": "Осталось",