[Docs] ENVIRONMENT.md lists invalid STELLAR_NETWORK values (pubnet/sandbox) and omits mainnet

[grantfox-oss]

Telegram (ask questions / claim the issue here first): https://t.me/+DOylgFv1jyJlNzM0

Why this matters

docs/ENVIRONMENT.md:15 documents STELLAR_NETWORK allowed values as (testnet, pubnet, sandbox), but parseNetwork() in backend/src/config/stellar.ts:27-36 accepts only testnet or mainnet and throws on anything else. The doc omits the only other valid value (mainnet) and lists two values that crash startup.

Acceptance criteria

• Change the documented allowed values to testnet / mainnet
• Match the description to stellar.ts behaviour

Files to touch

• docs/ENVIRONMENT.md
• backend/src/config/stellar.ts

[goldandrew]

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

Hi Maintainer, I’d like to take this issue.

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

[grantfox-oss]

🦊 GrantFox — @goldandrew has been assigned to this issue!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #1211)
2. Your PR will be reviewed by the LabsCrypt maintainers

Good luck! Track your progress on GrantFox.

[drips-wave]

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

🧑‍💻 @goldandrew: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

🌊 Happy Wave 🌊