[vault-contract] Improve error handling and type safety (low severity)

[Villarley]

Based on SECURITY_AUDIT_VAULT_CONTRACT-V1.0.0.md (findings F-11, F-12, F-13).

Severity: Low

Summary

Validate non-negative amounts, unify getter behavior, and make preview_claim return Result.

Acceptance criteria

• Validate roi_percentage >= 0 and amount inputs in constructor (F-11)
• Unify unwrap_or vs expect policy across getters (F-12)
• Change preview_claim to Result<ClaimPreview, ContractError> or document assumptions (F-13)
• Tests pass