[S4] Add environment variable validation to the frontend on startup

[Akshola00]

Description

The frontend relies on NEXT_PUBLIC_API_BASE_URL being set at build time, but if it is missing the app silently makes requests to undefined which produces confusing network errors.

Add a startup check that validates all required public env vars and throws a clear error during build if any are missing.

Deliverables

• Create lib/env.ts that reads and validates required env vars (NEXT_PUBLIC_API_BASE_URL, NEXT_PUBLIC_STELLAR_NETWORK)
• If a required var is missing, throw with a message like: Missing required env var: NEXT_PUBLIC_API_BASE_URL
• Import and call the validation in lib/api-client.ts so it fails at module load time, not silently at runtime
• Add a .env.example file at nevo_frontend/ listing all required vars with placeholder values
• npm run build still passes with the vars present

Notes

No new dependencies needed — plain TypeScript is fine. Do not use zod unless it is already in the project dependencies.

[MJ-RWA]

@MJ-RWA has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hell web3Novalabs, can i work on this to implement this fix?
ETA: 6 hours!

ℹ️ Repo Maintainers: To accept this application, review their application or assign @MJ-RWA to this issue.

[RemmyAcee]

@RemmyAcee has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I've reviewed the task and have a clear implementation plan. Looking forward to contributing with a quality PR.

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[OtowoSamuel]

@OtowoSamuel has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I would love to work on this!

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Congratulations, @OtowoSamuel! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

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This issue has been completed by @OtowoSamuel as part of the Stellar Wave Program's 6th Wave 🥳

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