feat: system messages, group DM welcome header, and dm_owner_updated event

- 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

packages/web/src/hooks/useWebSocket.ts

@@ -814,6 +814,12 @@ function handleEvent(origin: string, event: ServerEvent): void {
       break;
     }
 
+    case 'dm_owner_updated': {
+      const { updateDmOwner } = useSpaceStore.getState();
+      updateDmOwner(event.dmChannelId, event.newOwnerId);
+      break;
+    }
+
     // ─── Channel/space events (all origins) ─────────────────────────────────
 
     case 'channel_created': {