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/dm.ts

@@ -1214,6 +1214,15 @@ export async function dmRoutes(app: FastifyInstance): Promise<void> {
           .where(eq(schema.dmChannels.id, id))
           .run();
 
+        // Broadcast ownership change via dedicated event
+        for (const member of remainingMembers) {
+          connectionManager.sendToUser(member.userId, {
+            type: 'dm_owner_updated',
+            dmChannelId: id,
+            newOwnerId: nextOwner.userId,
+          });
+        }
+
         // Query new owner user outside federation block so it's available for system message
         const newOwnerUser = db.select().from(schema.users).where(eq(schema.users.id, nextOwner.userId)).get();