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/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"