[Contracts] Add dispute resolution flow

[Kaylahray]

Description

The contract README explicitly calls out dispute resolution as a future enhancement, but the on-chain flow currently ends at donation, refund, or cancellation.

User Story

As a donor or requester,
I want to escalate a disputed help request or giveaway,
so that the contract can record a formal resolution path.

Requirements and Context

• Files: contracts/geev-core/src/mutual_aid.rs, contracts/geev-core/src/giveaway.rs, contracts/geev-core/src/test.rs
• Add a dispute state and a resolution function
• Restrict resolution to admin or authorized moderators
• Ensure a disputed request cannot continue normal payout flow until resolved

Suggested Implementation

// DISPUTE FLOW
// 1. Mark the request or giveaway as disputed.
// 2. Block normal completion or refund paths while disputed.
// 3. Add an admin resolution action.
// 4. Emit dispute lifecycle events.

Acceptance Criteria

• Disputed items can be marked on-chain.
• Normal completion or payout paths are blocked while disputed.
• Admin resolution changes the final state explicitly.

Submission Guidelines

• Branch: feat/dispute-resolution
• Depends on: request and giveaway state machine
• PR: feat(contracts): add dispute resolution lifecycle

[innovativedesign-lab]

@innovativedesign-lab has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hi Maintainer! I’m interested in contributing to this task and can ensure timely delivery with clean implementation.

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