fix(play): make scene-image proxy opt-in — default deployers connect direct

b805b1d routed every scene <img> through fetch → Blob → createObjectURL to
kill QUIC progressive-paint, but in doing so added an *unconditional*
dependency on a CORS-adding proxy. That breaks the default deployment:
im.runware.ai sends no Access-Control-Allow-Origin, so a direct
fetch().blob() throws and the scene image silently fails to load for anyone
who hasn't stood up the Cloudflare Worker.

Restore the pre-b805b1d behavior as the *default* and make the proxy
strictly opt-in:

  - Direct path (no env set): preloadImage() warms the HTTP cache + decodes,
    then <img> uses the original https://im.runware.ai URL — as before
    b805b1d. No fetch().blob(), no CORS dependency: a fresh clone just works.
  - Proxy path (NEXT_PUBLIC_IMAGE_PROXY_URL set): fetch the proxied URL →
    Blob → createObjectURL, exactly as b805b1d, gaining the QUIC-immune
    HTTP/2 edge + atomic paint.

shouldProxy(url) gates the two paths: proxy only when a base is configured
AND the host is in NEXT_PUBLIC_IMAGE_PROXY_ALLOWED_HOSTS (default
im.runware.ai). data: / non-http / unknown-host URLs always take the direct
path. blobUrlCache + revoke logic is unchanged and safe for both paths
(revoke is a no-op on non-blob: URLs).

The Cloudflare Worker moves out of this repo into a standalone, one-click-
deployable project (infiplot-image-proxy) so the optional infra isn't
carried by every clone; .env.example and the READMEs link to it.

restore: preloadImage() helper deleted by b805b1d
add:     NEXT_PUBLIC_IMAGE_PROXY_ALLOWED_HOSTS (default im.runware.ai)
remove:  worker/ (moved to standalone repo)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

README.md

@@ -154,6 +154,10 @@ InfiPlot 会与四类模型供应商通信。**文本（Text）和视觉（Visio
 
 使用推荐的三件套时，每一幕场景的开销主要来自图像生成模型。FLUX.2 [klein] 9B KV 的图像大约 **$0.00078** 一张（1792×1024，4 步，亚秒级）；文本模型使用 `deepseek-v4-flash` 时，成本极低。逐拍点过一个场景是免费的。为了让切换瞬间完成，引擎还会预测式地生成那些你可能选、但最终可能没选的场景 —— 所以真实花费会比你实际看到的场景数略高一些。
 
+**4. 图片代理（可选）**
+
+默认浏览器直连图片供应商，无需任何配置 —— 留空 `NEXT_PUBLIC_IMAGE_PROXY_URL` 即可，完全不受影响。只有当你遇到图片「层层加载」（Chrome 在某些网络下 `ERR_QUIC_PROTOCOL_ERROR` 导致 PNG 逐行渲染）时才需要它：部署一个极小的 Cloudflare Worker，把图片改为服务端转发 + HTTP/2 原子返回。一键部署见 **[infiplot-image-proxy](https://github.com/zonghaoyuan/infiplot-image-proxy)**，然后把它给出的 `workers.dev` 地址填进 `NEXT_PUBLIC_IMAGE_PROXY_URL`。
+
 ---
 
 ## Roadmap