fix: admin promo groups - add default toggle, fix threshold reset to 0

- Added is_default toggle to promo group create/edit form
- Send 0 instead of null when clearing auto-assign threshold
- Added isDefault i18n keys for ru, en, zh, fa

src/locales/fa.json

@@ -2200,6 +2200,7 @@
         "rub": "روبل",
         "autoAssignHint": "0 = تخصیص خودکار نشود",
         "applyToAddons": "اعمال به خدمات اضافی",
+        "isDefault": "گروه پیش‌فرض",
         "cancel": "انصراف",
         "saving": "در حال ذخیره...",
         "save": "ذخیره"