feat: Improve environment variables handling

[kamdubiel]

Description

• Deleted next-runtime-env and replaced it with super simple next-public-env
• Updated the Environment Variables.md docs
• Splt the env validation to separate server (secret) and client (public) validators
• Changed the way .env* files are picked up by the frontend apps, it's now following Next.js docs order
• Added nonce and CSP rules
• Fixed some minor warnings / errors that I missed during last Next.js update, and once that were required after adding nonces

Screenshots/Videos

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[DarkoKukovec]

Nice!