File layout convention

<app>/
  index.html
  css/styles.css
  js/
    app.js       orchestration + tool handler
    realtime.js  WebRTC + Realtime API + tool schemas
    storage.js   domain persistence + mutations
    ui.js        DOM rendering (read-only)

Keep each file ≤ ~300 lines. If realtime.js grows beyond that, domain logic is leaking in — move it to app.js.