Fix JS inference API code snippet

[xenova]

Fixes the following issues:

1. JS code snippets missing "Content-Type": "application/json",, leading to an error
2. Align closer to python snippet (e.g., API_URL outside of query)
3. Indentation in certain nodes, e.g., here.
   

TODOs/questions

• Is there a way to preview the outputs to make sure it renders correctly?
• For some reason, the python snippet still outputs API_URL and headers if body is empty, so I've done the same here (for now)

[Wauplin]

I don't think we should change snipperFile since it doesn't send a json body but raw bytes from a file (typically an audio or image file).

[Wauplin]

If we define HEADERS here, shouldn't we reuse it in fetchSnippet?
To do that, you would need to define "json headers" and "non-json headers" separately and inject the correct one depending on the task.