frontend: handle env var configuration at one location in the code

[Stezido]

• [x] The feature was discussed in discussions of this repository.
• [x] I have searched the issues of this repository and believe that this is not a duplicate.

Description 💡

I cannot figure out immediatley where in the frontend code its configuration or environment variables are located or managed. If I search for process.env I can locate the different locations of environmentvariables used but it's better to have them as done in api/blockchain code in one place in a config file.