feat: deposit transaction builder for wallet

[precious-akpan]

Description

Implements POST /api/vault/deposit/prepare endpoint that builds unsigned Stellar/Soroban transactions for USDC deposits.

Changes

• ✅ Amount validator with 7-decimal USDC validation
• ✅ Transaction builder service for creating unsigned Stellar transactions
• ✅ Deposit controller with comprehensive validation and error handling
• ✅ POST /api/vault/deposit/prepare endpoint with authentication
• ✅ Complete API documentation

Key Features

• Non-custodial: Backend never signs transactions or holds user keys
• Validation: Strict amount format (7 decimals), network, and source account validation
• Error handling: Comprehensive error responses for all failure scenarios
• Documentation: Complete API docs with examples and usage guide

Testing

Tests have been written for:

• Amount validator edge cases
• Transaction builder functionality
• Deposit controller integration

Note: Tests require npm install to complete due to network issues during development.

Resolves

Closes #40

Amount Format

USDC amounts must have exactly 7 decimal places:

• ✅ Valid: "100.0000000"
• ❌ Invalid: "100.00", "100"

Example Usage

// 1. Prepare transaction
const response = await fetch('/api/vault/deposit/prepare', {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json',
    'x-user-id': userPublicKey
  },
  body: JSON.stringify({ amount_usdc: '100.0000000' })
});

const { xdr } = await response.json();

// 2. Sign with wallet
const signedXdr = await window.freighterApi.signTransaction(xdr);

// 3. Submit to network
const result = await server.submitTransaction(signedXdr);