Setup unit tests

[elianiva]

To keep our code reliable and more resilient against changes, we should write unit tests for every code we add. The coverage doesn't have to be 100% but it's good to have close to 100% coverage.

We can use Vitest and React Testing Library for the frontend.
For backend we can use Jest and Supertest that already comes with NestJS. See this page for details.

For now let's just focus on unit tests, no need to write integration/E2E.

TODO:

• Frontend unit test setup
• Backend unit test setup