Add lesson_state table and API endpoints (PUT/GET/DELETE /api/lesson-state) to persist in-progress lesson state (current exercise index, hearts, correct count). On lesson load, check for saved state and restore it. After each exercise advance, save state to the server. On lesson completion, delete the saved state. This prevents losing all progress on disconnect or page refresh. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>