perryvegehan/UPI_Without_Internet — reverse-engineered prompt

Build me a Java Spring Boot demo app that shows how offline UPI payments could move through a phone mesh when there is no internet.

I want a simple local dashboard where I can pick a sender, receiver, amount, and PIN, then inject a payment into a simulated mesh of phones. Add buttons to run gossip rounds so the payment hops between nearby phones, then let an internet connected bridge phone upload it to the backend. The backend should decrypt the payment, reject tampered or expired packets, make sure the same packet can only settle once, and update account balances and a transaction ledger.

This is just a demo, not a real banking app. It should run on one laptop with Java only, no external database or real Bluetooth needed. Please include clear tests that prove duplicate uploads at the same time only debit the sender once.