Clean up: In Frontend `classes`, `entities`, `services` and `repositories` leftover

`classes`, `entities`, `services` and `repositories` They all "kinda of" do the same thing. It's very confusing.

Looks like the `classes` are a leftover from FullStack implementation. In the FE/BE implementation it's replaced with `repositories`.

**Task:** Suggest a conceptionally consistent approach on how to structure our Frontend code such that it follows common conventions and best-practises used in Frontend Engineering.

This concept should be described in the Developer Documentation section of our docs.

**Task2:** Once the concept has been developed, please create follow-up tickets to modify our code base to follow this concept.