Add Stellar network client helper

[Errordog2]

Summary

• Add a server-only lib/stellar/client.ts helper that builds on the existing Stellar config helper.
• Centralize testnet/mainnet Horizon URLs, Soroban RPC URLs, network passphrases, and simple request builders.
• Cover default testnet, mainnet, env overrides, invalid network handling, and helper construction in Vitest.

Closes #3

Validation

• npm test -- lib/stellar/client.test.ts passed: 6 tests.
• npx eslint lib/stellar/client.ts lib/stellar/client.test.ts passed.
• git diff --check -- lib/stellar/client.ts lib/stellar/client.test.ts passed.
• Sensitive scan over changed files found no payment/account/private-key markers.

Existing upstream blockers observed

• npm run typecheck --if-present is currently blocked by unrelated existing project type errors outside these files.
• npm run build is currently blocked by existing prerender/runtime issues outside these files.

No payment details or account identifiers are included in this PR.

[Obiajulu-gif]

@Errordog2 resolve conflict and make sure you fix you lint conflict