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/fa.json

@@ -1333,6 +1333,8 @@
         "save": "ذخیره"
       },
       "stats": {
+        "title": "آمار کد تخفیف",
+        "notFound": "کد تخفیف یافت نشد",
         "totalUses": "کل استفاده‌ها",
         "today": "امروز",
         "remaining": "باقی‌مانده",