Tasks: Improve Visibility of Unchecked Task State with Consistent Checkbox Styling

The new colored checkboxes look great! However, it's now harder to distinguish which tasks are still unchecked at a glance, since the checkbox color matches the task label color. It would be helpful if the unchecked state had a consistent visual treatment—such as using a darker shade or reverting to the original black—so users can more easily spot incomplete tasks.