Code-split the apps/web bundle

**Context:** PR #28 ships every public screen statically imported by `App.tsx`. The production bundle is 550 kB (164 kB gzipped), which vite warns about at build time.

The cheapest fix is `React.lazy()` on each screen so each route splits naturally. The shell + home are the hot path; everything else can lazy-load.

**Acceptance:**

- [ ] No single chunk > 200 kB gzipped
- [ ] First-paint route (`/`) loads only the hero + shell-needed code
- [ ] Smoke tests still pass (lazy + Suspense boundary doesn't break the render harness)