fix(federation): resolve persistent unread indicator on federated DMs

Federated relay messages can have local snowflake IDs that don't match
chronological (createdAt) order — a message sent earlier on a remote
instance can arrive later and get a higher local ID. This caused a
permanent mismatch between the ready event's lastMessage (MAX id) and
the acked message (last in createdAt display order), making federated
DM channels appear unread after every server restart.

- Server: change ready event DM lastMessage query from MAX(id) to
  ORDER BY created_at DESC (matching the DM REST API)
- Frontend: change ackChannel to ack MAX(id) among loaded messages
  instead of last in display order (consistent with server comparison)
- Fix federated username display fallback in UserDiscoverCard

packages/web/src/components/chat/FriendsPage.tsx

@@ -625,8 +625,8 @@ function UserDiscoverCard({
     username: string;
   } | null>(null);
 
-  const displayName = user.displayName ?? user.username;
   const baseName = user.username.includes('@') ? user.username.split('@')[0]! : user.username;
+  const displayName = user.displayName ?? baseName;
   const gradient = getAvatarGradient(user.homeUserId ?? user.id, displayName, user.avatarColor);
   const originLabel = user._instanceOrigin
     ? (() => { try { return new URL(user._instanceOrigin).host; } catch { return user._instanceOrigin; } })()