A dead authelia upstream almost always means "no users yet" — authelia
treats an empty user store as a fatal startup error, so an enabled but
unbootstrapped swarm crash-loops behind a vhost that is working
perfectly. nginx's default 502 says the opposite: it points at the
proxy, which is the one component that is fine.
Adds `ssoUnavailable` to the shared error-page set and wires it on the
authelia vhost the same way the per-agent blocks wire
`__hive_agent_unreachable`: `proxy_intercept_errors on` plus an internal
location serving the static page.
The page leads with the bootstrap command rather than burying it under
an explanation, and names the container journal as the fallback for the
cases where users are not the problem. Same Catppuccin template as its
siblings, so this costs no new styling.
