feat: Solidity escrow contract implementation (#1)

[Sycosmile]

feat: Solidity escrow contract implementation

Closes #1

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Summary

Implements the core GrantStream.sol escrow contract as described in issue #1. Funders can lock ETH across multiple milestones in a single transaction; each tranche is only released when the grantee submits IPFS evidence and the designated verifier approves it on-chain.

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What's included

contracts/GrantStream.sol

• createGrant() — funder locks ETH for all milestones in one tx, emits GrantCreated
• submitEvidence() — grantee attaches an IPFS URI per milestone (ipfs://Qm...)
• approveMilestone() — verifier releases that milestone's ETH to the grantee
• rejectMilestone() — verifier rejects; grantee can resubmit evidence
• cancelGrant() — funder cancels and receives refund of all unreleased funds (blocked while any milestone is SUBMITTED to prevent griefing)
• Auto-completes the grant when the last milestone is paid
• Gas-efficient custom errors instead of revert strings
• Full event log (GrantCreated, EvidenceSubmitted, MilestoneApproved, MilestoneRejected, GrantCancelled, GrantCompleted) — ready to feed the Activity Feed UI

test/GrantStream.test.js

• 24 test cases (Hardhat + Chai)
• Covers: grant creation, ETH locking, evidence submission, approval/rejection flow, resubmission after rejection, cancellation with partial refund, and all access control reverts

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Milestone status flow implemented

PENDING → (grantee submits) → SUBMITTED
       → (verifier approves) → APPROVED  → ETH sent to grantee
       → (verifier rejects)  → REJECTED  → grantee resubmits

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How to test

npm install
npx hardhat test

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Notes

• Targets Base Sepolia (chain ID 84532) as per the project README
• Solidity ^0.8.20 — compatible with the Hardhat config
• Evidence URIs are stored on-chain as strings; ready to integrate with the IPFS gateway (VITE_IPFS_GATEWAY) from .env.example
• getGrantsByFunder() and getGrantsByGrantee() view helpers are included to support frontend querying without indexing

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Happy to make any adjustments based on review. Let me know if you'd like the verifier role extended to support multisig (for Issue #10 down the line).

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