I need a full-stack Web3 engineer to wire a complete crypto payment gateway into my existing product. Everything will run on Ethereum and must be handled through Web3.js — from the smart-contract calls to the front-end hooks. Here’s what I expect: • A lightweight, gas-efficient contract that can receive and confirm payments. • A Web3.js integration that prompts the user’s wallet, tracks confirmations, and returns clear success or failure states back to the UI. • Basic test coverage and a short README so I can redeploy the contract and run the integration on another network. • A live demo on an Ethereum testnet showing a full payment round-trip. Once I can click a button, pay with ETH, and instantly see the transaction reflected in the app, the job is done.