Improve test coverage

[grimsi]

Since Gameyfin started as a personal project of mine to solve a very specific problem only I seemed to have I just developed it and did not write any tests (not even simple unit tests). Now that more people seem to be interested in my work I will need to implement at least some basic tests before I can start to work on new features. The goal of this issue is to have unit test coverage for both frontend and backend. In a future issue I will explore if it's necessary to also implement E2E-Tests with Selenium and/or Testcontainers or other suitable technologies.