don't rely on `inert` attribute (yet)

[dwhieb]

The inert attribute is:

• not supported on Firefox or Safari yet
• not accessible in the way you're trying to use it with checkboxes (users can't focus on it, so they can't tell its state)

Possible solutions:

• Find a way to style disabled checkboxes.
• Use pointer-events: none;.
• Create custom styles for checkboxes.
• Just use the default disabled styles.