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/server/src/routes/federation.ts

@@ -1972,6 +1972,12 @@ function processOwnershipTransferEvent(
     .where(eq(schema.dmChannels.id, channel.id))
     .run();
 
+  connectionManager.sendToDmMembers(channel.id, {
+    type: 'dm_owner_updated',
+    dmChannelId: channel.id,
+    newOwnerId: newOwnerLocal?.id ?? channel.ownerId!,
+  });
+
   const prevOwnerLocal = event.ownership.previousOwner
     ? resolveLocalUser(event.ownership.previousOwner.homeUserId, db)
     : null;