Warn on unanswered questions at submit; clarify blank results

• Submit now counts unanswered questions, highlights them, and asks for confirmation before submitting (they're scored incorrect).
• Results view shows 'You didn't answer this question' for blanks instead of a green-styled correct-answer line that read like a pass.

Note: scoring was already correct (blanks graded incorrect); this fixes the misleading UI and the missing submit-time warning.