fix(ui): a signed-in person is never shown the sign-in form

The router rendered `#/login`, `#/register` and `#/reset` before it checked
for a user. A browser signed in as one account, opening on a `#/login` left
in the address bar, drew the login form (prefilled by the browser with
another account) under a navigation bar and a sidebar already showing the
first account and its Administration entry.

`#/login` and `#/register` now need no one signed in, and a signed-in person
landing on either is sent home with `location.replace`, so Back does not lead
to the form again. `#/reset` stays reachable: that flow signs in half-way and
still has its progress and result to show.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T6jPTeocXA1BePekdsgPya

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