Routing broken when serving statically built SPA sites with `elide serve`

[franklinfollis]

Summary

Direct navigation to SPA routes returns 404 when serving a built React app with elide serve, even when the route should exist.

Steps to reproduce

1. Build a React app with client-side routing (e.g., React Router/Tanstack Router)
2. Serve the static files: elide serve ./dist
3. Paste a URL going to an existing route (like http://localhost:8080/my/route)
4. See 404 in browser console

Loading the root path (http://localhost:8080/) and navigating through to different routes from there works fine.