`ServeDir::not_found_service` wraps the fallback in `SetStatus`, which forces
every response to `404 Not Found`. Client side routes such as `/note/<id>` and
`/about` were therefore served the correct `index.html` but with a 404 status.
Use `ServeDir::fallback` instead, which leaves the status untouched. A note that
genuinely does not exist is still reported as 404 by `/api/notes/<id>`.
Behind a reverse proxy this made effectively every document request show up as a
4xx, skewing error rate dashboards and triggering false alerts.
Fixes#217
cupcakearmy
Matheus Leal