Add Stellar testnet/mainnet environment switching without code changes

[robertocarlous]

Summary

Switching between Stellar testnet and mainnet currently requires changing multiple environment variables manually with no validation that the full set is consistent. A misconfiguration (testnet key against mainnet RPC) could be catastrophic.

Proposed Solution

• Add a single STELLAR_NETWORK env var: testnet | mainnet | futurenet
• In src/stellar/client.ts, derive the RPC URL, passphrase, and explorer URL from this single value (with env var overrides for custom nodes)
• Add startup validation in src/config/readiness.ts that verifies the keypair matches the expected network
• Log the active network prominently at startup

Acceptance Criteria

• STELLAR_NETWORK=testnet sets all Stellar config correctly
• Startup validation rejects keypair/network mismatch with a clear error message
• .env.example updated; individual Stellar URL vars become optional overrides
• Unit tests verify derived config for each network value

[aymide1ee]

@aymide1ee has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hello, I would love to work on this issue please assign to me.

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[osasfaith]

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

I would love to work on this issue.

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

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[grantfox-oss]

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Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #238)
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