Add env-based API URL and contract IDs

[joelpeace48-cell]

Read API base URL and (optional) rewards/campaign contract IDs from VITE_* env and use them in fetch and Soroban calls. Document in frontend README.

[devxsameer]

@devxsameer has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

I’d like to take this issue. I’m a backend-leaning full stack developer with experience managing environment-based configuration, and I can integrate VITE env variables for API URLs and contract IDs, update usage across the app, and document everything clearly in the frontend README.

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

[D2758695161]

Crab Bounty Claim

Submitted PR #134 for this issue. Payment address: 0xaae0101ac77a2e4e0ea826eb4d309374f029b0a6