Closes #2364

Summary

• support Promise results from Modal OK handlers when loading is enabled
• close the modal when the Promise resolves
• stop the OK button loading state and keep the modal open when the Promise rejects
• update ModalConfig typings and add unit coverage

Tests

• pnpm exec eslint src/components/modal/modal.vue src/components/modal/confirm.js test/unit/specs/modal.spec.js
• pnpm run unit (blocked: old Karma setup cannot load webpack/sourcemap preprocessors in this local pnpm environment)