add comments, minor adjustments

frontend/src/components/CustomListElement.vue

frontend/src/components/LoginPage.vue

frontend/src/components/MainPage.vue

frontend/src/router/index.js

src/main/java/com/cloudcomputing/todo/entity/Todo.java

src/main/java/com/cloudcomputing/todo/entity/User.java

src/main/java/com/cloudcomputing/todo/mapper/TodoMapper.java

src/main/java/com/cloudcomputing/todo/mapper/UserMapper.java

...va/com/cloudcomputing/todo/repository/UserRepository.java

...ain/java/com/cloudcomputing/todo/service/UserService.java

...ava/com/cloudcomputing/todo/util/CustomAuthenticator.java