Quiz: optional reset when the participant leaves the quiz window #1731

Problem

An operator running a quiz has no way to discourage looking answers up in another tab.

Design

• Quiz setting "Reset the quiz when the participant leaves the window" (off by default).
• When on, a notice above the form says so before the participant starts.
• Trigger: visibilitychange to hidden (tab switch, minimise), not blur — a system notification, a screen reader or a click inside an embedded video fires blur without the person leaving.
• On return: the answers are cleared and an overlay explains why and offers to reload.
• It is a deterrent, not a security boundary (another device, JS disabled), and the documentation says so.
• Leaving does not count as an attempt. The number of exits is recorded on the submission (_quiz_focus_losses) and shown to the operator.

Acceptance criteria

• Setting, notice, overlay (accessible alertdialog), dark mode.
• Exit count stored and shown on the submission screen.
• Documentation, three catalogues, CHANGELOG, Vitest + PHPUnit tests.