Unexpected token "T" when trying to parse JSON comes from trying to do response.json() on a failed request

You should check the status code of the fetch before trying to parse it

In this case, it's getting a 404 and the following content, then erroring on the first letter T

1The page could not be found23NOT_FOUND45cle1::7ggdq-1764181414679-8a2ab6d6ee56

So the core issue is that /api/signin is not a page. If you go to your latest deployment in the dashboard and look under "Deployment Summary" it only shows static files and no serverless functions, so your vercel.json configuration is making it not deploy the /api routes

When this is working correctly, you'll see another folder called "Serverless Functions" right below static assets

Vercel has native support for the /api route with functions declared in this way, so you can actually remove the vercel.json entirely and it should pick them up

https://vercel.com/docs/functions/quickstart?framework=other