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"