Add UI-focused tests for extracted helper modules

[suspiriaa]

Summary

Expand automated coverage for helper modules extracted from ui.py, especially selection and storage logic.

Motivation

The project already has baseline regression tests, but the UI helper modules still have limited direct coverage.

Scope

• add tests for ui_selection.py
• add tests for ui_storage.py
• verify action editing flows such as delay updates and delete behavior
• verify serializer-backed load/save helpers handle success and validation errors correctly

Acceptance Criteria

• helper module tests run in pytest
• failure cases are covered as well as happy paths
• no existing test coverage regresses