[fix] don't use client-side router for same path on different origin

[Conduitry]

Fixes #4430. The special handling for 404s in SPA mode was erroneously kicking in when we were linking to the same path on a different origin. This resulting in SvelteKit trying to do client-side navigation, which failed when the history API refused to pushstate a URL with a different origin.

I didn't add a test, and may not have time to today. If someone else wants to write one and push to this branch, feel free.

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