- Render system messages (member_added, member_removed, owner_changed) inline
in MessageList with icon + human-readable text; system messages never group
with adjacent user messages
- Rewrite WelcomeHeader to branch on ownerId: group DMs show overlapping avatars,
group name, creator attribution, federated privacy note, and a Leave Group button
- Add dm_owner_updated ServerEvent; broadcast from dm.ts leave handler and
federation processOwnershipTransferEvent so all clients update ownerId in real-time
- Add updateDmOwner action to spaceStore and handle dm_owner_updated in useWebSocket
- Add leaveGroup alias to API client dm namespace