Check-in no longer goes blank when your session hiccups

If your sign-in quietly expired while a page was open — most visibly on
check-in, which refreshes itself constantly — the next button you pressed made
the screen go white. You had to reload and navigate back, then it happened
again on the next click.

The page was never broken and nothing you did was lost. Behind the scenes the
app was answering the button press with "go and sign in again" instead of
"here's the result", and the browser had no way to show you that. It now
returns a normal error, so you get a message telling you the session expired
and to reload — once — instead of a blank page every time.

Two related repairs: an expired sign-in is now properly cleared the first time
it is detected (previously it was re-detected on every request, which is why it
kept recurring), and a roster row can no longer blank the list if part of a
client's details is unavailable.

**One access correction.** A person's profile carried the role they held at the
venue where their account was first created, even after that changed. For nine
people this said "admin" of a venue where they are actually an instructor. Roles
are read from each venue's own team list, so this was never how their day-to-day
access was decided — but the stale value was a fallback that could have granted
an admin panel that was not theirs. Every profile now matches the venue's team
list, and nobody lost access to their own venue.