Add spell practice, memory analytics, auth persistence, and word library UX.

Includes per-word training stats and curves, quiz session auto-save, remember-login,
paginated word list with floating page arrows, and Obsidian-style relationship graph baseline.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/routers/quiz_router.py

@@ -24,6 +24,15 @@ def daily_quiz(
     return DailyQuizResponse(**result)
 
 
+@router.get("/spell", response_model=DailyQuizResponse)
+def spell_quiz(
+    db: Session = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+):
+    result = quiz_service.get_spell_quiz(db, current_user)
+    return DailyQuizResponse(**result)
+
+
 @router.post("/answer", response_model=QuizAnswerResponse)
 def submit_answer(
     data: QuizAnswerRequest,
@@ -37,6 +46,7 @@ def submit_answer(
         data.question_type,
         data.user_answer,
         data.correct_answer,
+        data.duration_seconds or 0,
     )
     return QuizAnswerResponse(**result)