[Feature] Integrate Soroban milestone approval transactions (Closes #155)

[jaymoneymanxl]

Overview

Integrates the TaskChain frontend with Soroban smart contracts to approve milestone completion on-chain. This ensures milestone approvals are cryptographically secure and recorded on the Stellar network, providing transparency and immutability for escrow releases.

Changes

📦 New: lib/soroban/milestone-approval.ts

Soroban Transaction Service — Builds, signs, submits, and tracks milestone approval transactions:

Build Transaction:

• Constructs a Soroban contract call to approve_milestone(milestoneId)
• Uses @stellar/stellar-sdk TransactionBuilder with proper Soroban data
• Pre-simulates via prepareTransaction() to get accurate fee and footprint

User Signature:

• Accepts a signTransaction callback for wallet integration (Freighter, Albedo, etc.)
• Returns the unsigned XDR for the wallet to sign
• Supports any wallet that provides an XDR signing interface

Submit Transaction:

• Sends the signed XDR to the Soroban RPC endpoint
• Handles network errors gracefully with descriptive messages

Transaction Status:

• Real-time status updates via onStatusChange callback
• States: idle → building → awaiting_signature → submitting → pending → confirmed | failed
• Polls for confirmation with configurable timeout (60s default)

Type Safety:

• Full TypeScript types for all states, params, and responses
• MilestoneApprovalState union for discriminated status handling
• MilestoneApprovalParams interface for type-safe configuration

Verification

cd TaskChain
npx tsc --noEmit lib/soroban/milestone-approval.ts
# Expected: No type errors

Integration Example

import { approveMilestoneOnChain } from "@/lib/soroban/milestone-approval"

const { status, txHash } = await approveMilestoneOnChain({
  clientAccountId: "GABCD...",
  contractId: "CC...",
  milestoneId: "milestone_123",
  networkPassphrase: Networks.TESTNET,
  rpcUrl: "https://soroban-testnet.stellar.org",
  signTransaction: async (xdr) => window.freighter.signTransaction(xdr),
  onStatusChange: (state) => setTxState(state),
})

Closes #155