Add docs for client environment variables and their effect on runtime behaviour

[Chucks1093]

Summary

The client reads several environment variables at build time but none of them are documented. New contributors have to grep the codebase to find which variables exist and what they control. A reference doc should list every variable with its name, purpose, required/optional status, and default value.

Scope

• List all VITE_ environment variables consumed by the client
• For each: variable name, description of what it controls, whether it is required or optional, default value if optional
• Add a note on how to set up a local .env file from the .env.example
• Document which variables are build-time only versus referenced at runtime

Acceptance Criteria

• All VITE_ variables listed with name, description, required/optional, and default
• Build-time vs runtime distinction noted
• .env setup instructions included
• Document lives in docs/ and is linked from the local setup guide

ETA: 12 hours

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Coordinate on Telegram

[MoscowDev]

I’d love to take this issue on. I’ve worked on production-ready applications and collaborated extensively through GitHub workflows. I’ll dig into the current setup, implement a solid and reliable fix, and deliver a well-tested PR. My focus is on clean, maintainable code and fast, dependable delivery.