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/store/uiStore.ts

@@ -18,6 +18,7 @@ export interface UIState {
   suggestionSidebarOpen: boolean;
   aiSidebarCollapsed: boolean;
   aiSidebarTab: AISidebarTab;
+  messageSidebarCollapsed: boolean;
   toasts: Toast[];
   notifications: string[];
   setTheme: (theme: Theme) => void;
@@ -29,6 +30,8 @@ export interface UIState {
   setAISidebarCollapsed: (collapsed: boolean) => void;
   setAISidebarTab: (tab: AISidebarTab) => void;
   openAISidebarProactive: () => void;
+  toggleMessageSidebar: () => void;
+  setMessageSidebarCollapsed: (collapsed: boolean) => void;
   addToast: (toast: Omit<Toast, 'id'>) => void;
   removeToast: (id: string) => void;
   clearToasts: () => void;
@@ -45,12 +48,9 @@ export const useUIStore = create<UIState>((set) => ({
   suggestionSidebarOpen: false,
   aiSidebarCollapsed: true,
   aiSidebarTab: 'chat',
+  messageSidebarCollapsed: true,
   toasts: [],
-  notifications: [
-    'Neuer Kontakt hinzugefügt: Max Mustermann',
-    'Firma aktualisiert: TechCorp GmbH',
-    'Meeting um 14:00 Uhr mit Müller AG',
-  ],
+  notifications: [],
   setTheme: (theme) => {
     localStorage.setItem('leocrm_theme', theme);
     set({ theme });
@@ -66,6 +66,8 @@ export const useUIStore = create<UIState>((set) => ({
   setAISidebarCollapsed: (collapsed) => set({ aiSidebarCollapsed: collapsed }),
   setAISidebarTab: (tab) => set({ aiSidebarTab: tab }),
   openAISidebarProactive: () => set({ aiSidebarCollapsed: false, aiSidebarTab: 'proactive' }),
+  toggleMessageSidebar: () => set((s) => ({ messageSidebarCollapsed: !s.messageSidebarCollapsed })),
+  setMessageSidebarCollapsed: (collapsed) => set({ messageSidebarCollapsed: collapsed }),
   addToast: (toast) => {
     const id = `toast-${++toastIdCounter}`;
     set((s) => ({ toasts: [...s.toasts, { ...toast, id }] }));