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/en.json

@@ -2538,6 +2538,7 @@
         "rub": "rub.",
         "autoAssignHint": "0 = don't auto-assign",
         "applyToAddons": "Apply to additional services",
+        "isDefault": "Default group",
         "cancel": "Cancel",
         "saving": "Saving...",
         "save": "Save"

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"

src/locales/ru.json

@@ -3059,6 +3059,7 @@
         "rub": "руб.",
         "autoAssignHint": "0 = не назначать автоматически",
         "applyToAddons": "Применять к дополнительным услугам",
+        "isDefault": "Группа по умолчанию",
         "cancel": "Отмена",
         "saving": "Сохранение...",
         "save": "Сохранить"

src/locales/zh.json

@@ -2199,6 +2199,7 @@
         "rub": "卢布",
         "autoAssignHint": "0 = 不自动分配",
         "applyToAddons": "应用于附加服务",
+        "isDefault": "默认组",
         "cancel": "取消",
         "saving": "保存中...",
         "save": "保存"