[fix] make demo app work without JS

[geoffrich]

Fixes #3952

After the introduction of shadow endpoints, the demo app would throw a "Body returned from shadow endpoint request handler must be a plain object" error when adding a todo with JS disabled. This was because we were returning a Response, not a POJO.

After fixing that issue, I also had to modify the request passed to the api in the get handler. When rendering a page in response to a POST, the get handler is called with a POST request. Since we passed the original request to the api method, we were sometimes making a POST request when we needed a GET. By making a new request, we make sure the method is GET in all cases. I added an explanatory comment to the code since it's a little weird.

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[geoffrich]

As an aside, I didn't see the "call the GET handler after POST when rendering a page" behavior captured in the docs. Should it be?

[Conduitry]

There's a code comment in https://kit.svelte.dev/docs/routing#endpoints-post-put-patch-delete of "The page always has access to props from get..." but that could maybe have a bit of a bigger deal made of it.

[Rich-Harris]

I had some suggestions but it was easier to try them out and submit an alternative PR, rather than try to cram them into inline commit suggestions: #3970

[geoffrich]

Closing in favor of #3970.