Add avatar/name/username identity card in the federation connect phase
so users can confirm their account before entering their password.
Promote "I use another instance" from a hidden <details> collapsible to
an equal-weight button with an "or" divider, using a dedicated
'other-instance' phase with Back navigation.
Make /join/:code a public route with a standalone JoinPage that shows a
space preview and handles authenticated, unauthenticated, and cross-instance
users. Adds GET /api/spaces/invite/:code/preview (no auth) endpoint,
?redirect= param support on login/register, and cleans up dead invite
handling from AppLayout and JoinSpace modal.
Jannis Braun