Issue 002: Split app structure

[cody257]

Summary

• Split the old monolithic src/App.jsx into page components for landing, buyer, and seller screens.
• Added a reusable Toast component.
• Kept existing behavior and styling intact so future UI/design work has cleaner boundaries.

Verification

• npm run lint
• npm test
• npm run build

Notes

• This is a mechanical structure change for Issue 002. It intentionally does not redesign the app or add routing yet.