Transaction 추가 및 로그인, 회원가입 validate 적용, backend api 라우터 정리

backend/models/Book.js

backend/routes/books.js

backend/routes/emails.js

backend/routes/login.js

backend/routes/transaction.js

frontend/package-lock.json

frontend/package.json

frontend/src/components/BookSearchPage.vue

frontend/src/components/Buy.vue

frontend/src/components/Sell.vue

frontend/src/components/home_hotlist/Hotlist.vue

frontend/src/components/login/Login.vue

frontend/src/components/login/SignUp.vue

frontend/src/components/mainpage.vue

frontend/src/components/profilecomponent/buylist.vue

frontend/src/components/profilecomponent/sellist.vue

frontend/src/router/index.js