feat: Unified Messaging System — kommunikation plugin, AI/Proactive/System participants, MessageSidebar, Rich Content Renderer

Phase 1: Backend plugin kommunikation (13 files, 10 tables, REST API, WebSocket, RBAC, DMS Bridge, Participant Registry, Mini-App Registry, Search Provider)
Phase 2: AI plugins as participants (ai_assistant + ai_proactive dock as participants, heartbeat job)
Phase 3: system_notif plugin (system events → chat messages, pinned System room)
Phase 4: Frontend MessageSidebar (replaces AISidebar, same design, comm API client, WebSocket hook, commStore)
Phase 5: Rich Content Block Renderer (11 components: Markdown, HTML, Image, Audio, Video, File, ActionCard, ContactCard, MiniApp, BlockRenderer)
BasePlugin: added services property + _container in on_activate

frontend/src/components/layout/TopBar.tsx

@@ -14,7 +14,7 @@ export function TopBar() {
   const navigate = useNavigate();
   const { user, currentTenant } = useAuthStore();
   const tenants = user?.tenants || [];
-  const { toggleSidebar, openAISidebarProactive } = useUIStore();
+  const { toggleSidebar, toggleMessageSidebar } = useUIStore();
   const logoutMutation = useLogout();
 
   const [userMenuOpen, setUserMenuOpen] = useState(false);