Offer GitHub Pages-hosted embedded store for testing

To improve onboarding and testing the store:

In case people don't have an easy way to upload HTML files for their store, we could put a template HTML page to GitHub pages. Then users would need to "whitelist" github.com with FastSpring, and that's it.

The package would need to inject the `STORE-ID` and `PRODUCT-ID` into the HTML in that case, though. Might as well inject 100% of the HTML into an empty page in that case.

首先檢視 package 目前對 store HTML 的處理方式，以及託管在 GitHub Pages 上的 template 如何適配 macOS component。定義注入 STORE-ID、PRODUCT-ID 和頁面內容所需的行為，然後確定 onboarding 和測試將如何驗證託管流程。

由索引模型根據 Issue 內容生成。