validate required environment variables at startup

[Harushii16]

Fixes #
Adds startup validation for required environment variables.

✨ What’s New

• Added src/utils/env.js to validate key environment variables:
  • VITE_SUPABASE_URL
  • VITE_SUPABASE_ANON_KEY
  • (optional) MONGODB_URI
• Imported the validator at the top of src/main.jsx so checks run immediately before app mounts.
• If variables are missing or invalid, a clear red overlay appears in the browser with a helpful message.
• Prevents cryptic runtime errors and helps new contributors set up .env correctly.

🚀 Why This Change

This improves developer experience by ensuring missing or incorrect environment variables are caught early.
Contributors can quickly identify setup issues without digging through console errors.

🧪 How to Test

1. Run npm run dev
2. If .env is missing or values are invalid, you'll see a red overlay + console error.
3. Add proper values in .env (based on .env.example), restart — app starts normally.

🏷️ Hacktoberfest

This contribution is part of Hacktoberfest 2025 🎉
It’s a genuine enhancement that improves the project’s reliability and developer onboarding.
Kindly label this PR as hacktoberfest-accepted if it meets contribution guidelines.

✅ Summary

• Environment validation added
• Overlay UI for clear feedback
• No additional dependencies required
• Tested locally
• Ready for review