Fix missing env variables in server.js

[jakob-info]

The VITE_* variables from .env in /frontend/ are not available in server.js. The added package dotenv fixes that as it loads them into process.env. Fixes #486

Checklist

• I have read the contributing guidelines.
• My code is of good quality and follows the coding standards of the project.
• I have tested my changes, and they work as expected.

Thank you for your contribution! 🎉

[daveearley]

Thanks for this! This will be useful for people who aren't using the Docker setup