This repository demonstrates the issue with the fallback html file not being added to the
service worker manifest.
Just run pnpm run build and inspect the service worker code in build/service-worker.js to see if app.html (or app as
it seem to drop the .html at least in some cases) is present.
.svelte-kit/output/prerendered/pages only contain the two pre-rendered pages index.html and test.html but is
missing the app.html (or fallback.html as the plugin seem to expect).
Ether file is also missing in output/client.
However, it does exist in build after the build is finished.