UnitField
class UnitField<T>(val name: String, val keyName: String, val meta: SettingMeta?, getter: (T) -> Unit, setter: (T, Unit) -> T) : SettingField<T, Unit>
No-op field used for Button settings backed by Unit.
Not persisted (read() always null, write() does nothing)
Getter/setter exist only to satisfy SettingField