[frontend] "Page not found" page says user is logged out when they are logged in

[alxndrsn]

Per #1697.

Repro

1. open https://staging.getodk.cloud/projects/203/forms/trees_update/
2. observe "Not logged in" text at the top-right corner of the page:

Expected

User is logged in, so header should not say "Not logged in".