Export flashcards to Anki-compatible CSV

[aimanmalib]

LearnPath generates flashcards but they live in-app. An export to CSV (front,back columns — importable into Anki/Quizlet) would make them portable.

Steps

1. Add an export helper that serializes the current flashcard set to CSV.
2. Wire a download button into the flashcard UI.
3. Add a unit test for the CSV serializer (header + row escaping).

Self-contained — mostly a small lib function + a button.