Idle detection

\see https://web.dev/idle-detection/ \see https://developer.mozilla.org/en-US/docs/Web/API/IdleDetector

The Idle Detection API can detect inactive users and locked screens. JShelter either removes the API (IdleDetector), or disables permission requests and returns user activity and unlocked screen (for previously granted permissions), or just always report user activity and unlocked screen.

Known bug: JShelter does not block events, so the page is notified that something changed but it cannot read what changed.