Language flashcards with spaced repetition

Build a spaced-repetition flashcard app for language learning.

Core features:
1. Create decks with front/back (and optional audio)
2. SM-2 style review queue: due today
3. Rate answer (again/hard/good/easy) and reschedule
4. Stats: retention, cards mastered, streak
5. Import from CSV

Data: decks, cards, reviews.
Must-haves: keyboard shortcuts, dark mode, offline review.