Refactor: split lib.rs into types, errors, and contract modules

[henrypeters]

Body:
Category: Smart Contract - Refactor
Priority: Medium
Estimated Time: 2 hours

Description:
Both lib.rs files mix type definitions, storage keys, contract logic, and tests in a single file. Split into modules for maintainability.

Tasks:

• Create types.rs for structs and enums
• Create errors.rs for ContractError
• Move test module to tests.rs
• Update lib.rs to declare modules

[cybermax4200]

@cybermax4200 has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

I can deliver an excellent work on this

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[drips-wave]

Congratulations, @cybermax4200! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on March 30, 2026.

🧑‍💻 @cybermax4200: 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.

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