Show page thumbnails, lightbox, and reasoning on quiz cards

[nicpottier]

Summary

• Display page thumbnails at the top of each quiz card, showing the source pages for the quiz
• Add clickable lightbox to view full-size page images (with Escape or click-to-close)
• Display quiz reasoning in italics at the bottom of each card
• Thumbnails wrap to multiple rows if needed, with light borders and hover effects

Test plan

• Verify quizzes page loads with thumbnails visible
• Click a thumbnail to open lightbox, verify Escape key closes it
• Verify page reasoning displays at bottom of quiz cards
• Test that editing quizzes still works (save/discard functionality)