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/AppShell.tsx

@@ -3,7 +3,7 @@ import { Outlet, useLocation } from 'react-router-dom';
 import { Sidebar } from './Sidebar';
 import { TopBar } from './TopBar';
 import { PluginToolbar } from './PluginToolbar';
-import { AISidebar } from './AISidebar';
+import { MessageSidebar } from './MessageSidebar';
 import { ToastContainer } from '@/components/ui/Toast';
 import { useAIContext } from '@/hooks/useAIContext';
 
@@ -13,8 +13,8 @@ export function AppShell() {
   // Track context for AI Proactive suggestions
   useAIContext();
 
-  // Hide AI sidebar on the AI Assistant page itself
-  const showAISidebar = !location.pathname.startsWith('/ai-assistant');
+  // Hide message sidebar on the AI Assistant page itself
+  const showMessageSidebar = !location.pathname.startsWith('/ai-assistant');
 
   return (
     <div className="flex h-screen overflow-hidden bg-secondary-50" data-testid="app-shell">
@@ -34,8 +34,8 @@ export function AppShell() {
           </main>
         </div>
       </div>
-      {/* AI Sidebar — full height, right of TopBar and Toolbar */}
-      {showAISidebar && <AISidebar />}
+      {/* Message Sidebar — full height, right of TopBar and Toolbar */}
+      {showMessageSidebar && <MessageSidebar />}
       <ToastContainer />
     </div>
   );