[Copilot]

Students lose all answer selections if they navigate away from an in-progress exam before submitting. Answers now persist in localStorage and are restored automatically on return.

Changes

State storage (exam.html)

• getExamStateKey() — derives a localStorage key from the URL token (or attemptId as fallback); key format validated with [a-zA-Z0-9_-]+ pattern before use
• persistExamData() — writes exam metadata (examData + attemptId) to ${key}_meta once on render; not called on every answer change
• saveExamState() — writes only the lightweight {questionId → answerId} map to ${key}_answers on each radio change
• restoreExamAnswers() — re-checks saved radio inputs after render; validates IDs are integers before querySelector interpolation; expires state older than EXAM_STATE_EXPIRY_MS (7 days)
• clearExamState() — removes both _meta and _answers keys on successful submission

Integration points

• renderExam() calls persistExamData() then restoreExamAnswers() after building the question list
• onAnswerChange(skipSave?) accepts a flag to skip the write when called from restore (avoids redundant re-serialize)
• init() falls back to activeExamStateKey (with regex validation + schema check) when no token or currentExam is present, enabling recovery for non-token-URL exam sessions

// On every answer change — only the small answers map is written
function saveExamState() {
    localStorage.setItem(`${key}_answers`, JSON.stringify({ answers, savedAt: Date.now() }));
}

// On render — full exam data written once
function persistExamData() {
    localStorage.setItem(`${key}_meta`, JSON.stringify({ examData, attemptId, savedAt: Date.now() }));
    localStorage.setItem('activeExamStateKey', key);
}