[Rewrite-Frontend]: Demonstrating classes on the timetable

In the rewrite version of Notangles, even if you select a course, no class will be shown on the frontend. This is because the actual card components have not been added and related backend API should be called to retrieve classes.

To enable this functionality, related card components should be created as well as APIs of adding, selecting, and deleting classes in the frontend. The implementation can refer to the one on the dev branch.