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/auth_router.py

@@ -46,7 +46,7 @@ def login(data: UserLogin, db: Session = Depends(get_db)):
     if not user or not verify_password(data.password, user.password_hash):
         raise HTTPException(status_code=401, detail="用户名或密码错误")
 
-    token = create_access_token(user.id, user.username)
+    token = create_access_token(user.id, user.username, remember=data.remember)
     return TokenResponse(access_token=token)