Implement invoice deadline soft-lock during active dispute

[Kingsman-99]

Label: complexity: high
Points: 200

Description

If dispute arbitration is implemented (separate issue), an invoice's deadline could pass while a dispute is being arbitrated, triggering an unwanted automatic refund mid-arbitration. This issue freezes the deadline clock during an active dispute.

Technical Context

Involves lib.rs refund() — when checking env.ledger().timestamp() > invoice.deadline, also check a disputed flag (from the dispute arbitration feature) and block refund eligibility entirely while disputed, regardless of how far past the deadline the ledger time has advanced.

Acceptance Criteria

• refund() panics with "invoice under dispute" instead of processing while disputed == true, even past the deadline
• Once the dispute resolves (cleared), normal deadline-based refund eligibility resumes immediately
• Non-disputed invoices are completely unaffected by this change
• Test: disputed invoice past deadline blocks refund; resolving the dispute then allows refund to proceed
• All existing cargo tests pass
• cargo clippy passes with zero warnings

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[BigElv]

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

Hi maintainer I would love to work on this issue please

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[Komelove]

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

This issue aligns closely with my experience in contracts, backend architecture, and frontend implementation. I’m keen to work on it and will keep the team informed of my progress along the way.

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[johnsaviour56-ship-it]

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I would love to work on this issue

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[grantfox-oss]

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[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign campaign!

@johnsaviour56-ship-it's PR #254 was approved and merged by @Kingsman-99.

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