Fix environment variable fetching

Related issues

Closes #84 (closed)

Acceptance criteria

• Implemented code changes don't diverge from the related issue.
• Only released versions of dependencies are used (i.e. no snapshot versions).
• Pipeline still works.
• GitLab issue is up-to-date.

Proposed squash commit message

Fix environment variable fetching in environment.prod.ts

• The environment variables are all fetch that way: window['env']['BACKEND_URL'].
• The former way end up in some errors, that the property 'env' does not exist on type 'Window & typeof globalThis'.
• All change are only made in the angular-frontend/src/environments/environment.prod.ts file.