config_file_unsupported

%file% is no longer used. Please pass configuration via the sveltekit(...) plugin in your Vite config.

SvelteKit no longer reads svelte.config.js. Move your options into the sveltekit(...) plugin call in vite.config.js, then delete the old file:

import { sveltekit } from '@sveltejs/kit/vite';
import adapter from '@sveltejs/adapter-auto';
import { defineConfig } from 'vite';

export default defineConfig({
	plugins: [
		sveltekit({
			adapter: adapter()
		})
	]
});