docs: add Firebase environment variables to README setup guide

[Vaishnavi10706]

📌 Description

This PR updates the README.md to centralize frontend environment variable documentation by including all required Firebase configuration variables in the Frontend — frontend/.env section.

This improves the contributor onboarding experience by ensuring all required frontend environment variables are documented in one place, reducing confusion during local setup.

🔗 Related Issue

Closes #1421

🛠 Changes Made

• Added a complete table of required frontend environment variables under Frontend — frontend/.env
• Documented all required Firebase configuration variables alongside VITE_API_URL
• Added a reference to the existing Google Authentication Setup section for obtaining Firebase credentials
• Kept documentation changes limited to README.md only

✅ Checklist

• Code runs locally
• Followed project structure
• No console errors
• Properly tested changes
• Linked the issue

🚀 Notes for Reviewers

• This is a documentation-only update.
• No application code, configuration files, or project structure were modified.
• The change is focused on improving setup clarity for new contributors by centralizing frontend environment variable requirements.