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)

先阅读 App.tsx 以及 PR #28 引入的静态屏幕导入，然后运行现有的 production build 和 smoke tests，以建立基线。拆分各个屏幕，使 home route 只保留 shell 和 hero 代码，并验证没有任何 chunk 在 gzip 压缩后超过 200 kB，且 smoke tests 仍能成功渲染。

由索引模型根据 Issue 内容生成。