feat(admin): add drag-and-drop group sort order

- Add `sort_order` field to groups table with migration
- Add `PUT /api/v1/admin/groups/sort-order` API for batch update
- Implement drag-and-drop UI using vue-draggable-plus
- All queries now order groups by sort_order
- Add i18n support (en/zh) for sort-related UI text
- Update test stubs to satisfy new interface methods

backend/ent/runtime/runtime.go

@@ -409,6 +409,10 @@ func init() {
 	groupDescSupportedModelScopes := groupFields[20].Descriptor()
 	// group.DefaultSupportedModelScopes holds the default value on creation for the supported_model_scopes field.
 	group.DefaultSupportedModelScopes = groupDescSupportedModelScopes.Default.([]string)
+	// groupDescSortOrder is the schema descriptor for sort_order field.
+	groupDescSortOrder := groupFields[21].Descriptor()
+	// group.DefaultSortOrder holds the default value on creation for the sort_order field.
+	group.DefaultSortOrder = groupDescSortOrder.Default.(int)
 	promocodeFields := schema.PromoCode{}.Fields()
 	_ = promocodeFields
 	// promocodeDescCode is the schema descriptor for code field.