feat: real-time sync, notification & social system overhaul

- Add WS events: dm_channel_created, dm_channel_closed, friend_removed,
  channel_created/updated/deleted, server_updated
- Fix first-ever DM: broadcast dm_channel_created to recipient
- Add DELETE /api/dm/:id for closing DMs with re-open support
- Wire Close DM button in sidebar
- Fix dm_message_created for unknown channels (safety net)
- Broadcast friend_removed on friend deletion
- Sound system: track realtimeMessageEvents separately from API loads,
  play notification for messages in all channels, not just current
- Optimistic updates: send/edit/delete messages appear instantly with
  rollback on failure, temp message deduplication on WS echo
- Channel CRUD broadcasts to all server members
- Server update broadcast on PATCH
- Server join registers user in connectionManager immediately
- Reconnect: only reload current channel, preserve other channel caches
- Activity panel, right panel, member list toggle components

packages/web/src/components/layout/ChannelSidebar.tsx

@@ -208,7 +208,15 @@ export function ChannelSidebar() {
                     )}
                   </div>
                   <button
-                    onClick={(e) => { e.stopPropagation(); }}
+                    onClick={(e) => {
+                      e.stopPropagation();
+                      // Navigate away if currently viewing this DM
+                      if (currentChannelId === dm.id) {
+                        navigate('/channels/@me');
+                        setCurrentChannel(null);
+                      }
+                      useServerStore.getState().closeDm(dm.id);
+                    }}
                     className="opacity-0 group-hover:opacity-100 text-discord-text-muted hover:text-discord-text-primary transition-opacity flex-shrink-0 ml-1"
                     title="Close DM"
                   >