There’s also Routify which Rich Harris (Svelte and Sapper creator) mentioned favorably.
Sapper seems to be the easiest to get started with. There might be a few little gotchas with authentication because Sapper is server-side rendered.
You could also use routing from Elixir/Phoenix etc. and just include Svelte components in your pages.
There are a few libraries for such things, for example Sveltex.