fix: allow hosting hash apps from other html files

[lishaduck]

Fixes #14824.

I'll check off the other checklist items when I get a free second, but the patch itself is good AFAICT

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[lishaduck]

test-kit-cross-browser (18, windows-latest, chromium, build) looks like a flake, but CI / test-kit-server-side-route-resolution (build) (pull_request) looks like it might be genuine...

[lishaduck]

Locally, fail
[chromium-build] › test/cross-platform/client.test.js:386:2 › Scrolling › scroll is restored after hitting the back button
[chromium-build] › test/cross-platform/client.test.js:924:2 › Routing › responds to

submission without reload

I think they're flaky? scroll is restored after hitting the back button is related to hash routing, but shouldn't affected and isn't hit in CI; form submission is completely unrelated AFAIK.

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-14825-svelte.vercel.app/

[dummdidumm]

thank you!