Federated 1-on-1 DMs showed the raw snowflake ID as the display name
and no avatar when the remote user had no pre-existing local record.
processCreateEvent used resolveLocalUser (find-only) instead of
resolveOrCreateReplicatedUser, and relay events carried no profile data
for participants.
- Add profile snapshot (displayName, avatar, avatarColor) to
FederationRelayParticipant and populate it in getDmParticipants
- Change processCreateEvent to auto-create replicated user stubs and
hydrate them with profile data from the relay event
- Fix hydrateReplicatedUserProfile URL resolution for homeInstance
values without protocol prefix
- Fix WelcomeHeader: return null while DM data is loading (eliminates
"unknown" flash on reload), use displayName for @mention text