Build Halves, a web app for splitting shared expenses with roommates. It needs a landing page that explains the product, and the working app: add an expense (what it was for, the amount, who paid, and who it's split between), see the list of expenses, and see who owes whom using the fewest payments. Keep the data in localStorage so it's still there after a reload. Deliver one self-contained index.html. No external images; CDN libraries are fine.