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/RightPanel.tsx

@@ -0,0 +1,16 @@
+import React from 'react';
+import { MemberSidebar } from './MemberSidebar';
+import { ActivityPanel } from './ActivityPanel';
+import { useUIStore } from '../../stores/uiStore';
+import { useServerStore } from '../../stores/serverStore';
+
+export function RightPanel() {
+  const showDms = useUIStore((s) => s.showDms);
+  const currentServerId = useServerStore((s) => s.currentServerId);
+
+  if (showDms || !currentServerId) {
+    return <ActivityPanel />;
+  }
+
+  return <MemberSidebar />;
+}