This seems incredibly dumb to me, but the solution was to rename svelte.config.ts to svelte.config.js. No errors in my terminal made this clear to me, but I found this doc file: https://github.com/sveltejs/vite-plugin-svelte/blob/main/docs/config.md#config-file-resolving. Annoyingly, the sveltekit plugin for Vite doesn't allow you specify the config file (but the svelte plugin, which sveltekit wraps, does have this option). Seems like a massive oversight by the devs here.