feat(UI-Overhaul-Phase2): AI Assistent in Kommunikation integriert

Migration 0137: Drop AI chat tables (ai_chat_sessions, ai_chat_messages, ai_chat_attachments, ai_conversations, ai_messages)

Backend:
- Remove AIChatSession, AIChatMessage, AIChatAttachment models from ai_assistant/models.py
- Remove AIConversation, AIMessage from app/models/__init__.py
- Remove session/message/stream/attachment routes from ai_assistant/routes.py
- Add new streaming route POST /ai/conversations/{conversation_id}/stream using comm tables
- Add new messages route GET /ai/conversations/{conversation_id}/messages using comm tables
- Add stream_chat_comm, get_comm_messages, save_comm_message to services.py
- Update external_api.py to use CommConversation/CommMessage instead of AIChatSession/AIChatMessage
- Update unified_search ai_chat_provider to search comm_messages with conversation_type=ai
- Remove ai_copilot router from main.py and routes/__init__.py
- Remove ai_conversation from entity_permissions.py and owner_transfer_service.py
- Update ai_assistant/plugin.py get_entity_models to remove AIChatSession
- Guard ai_copilot_service.py imports with try/except

Frontend:
- Remove AIAssistant.tsx, AIAssistantStandalone.tsx, SessionList.tsx, ChatWindow.tsx
- Remove AI Assistant routes from routes/index.tsx
- Update api/ai.ts: streamChat uses /ai/conversations/{id}/stream, fetchMessages uses /ai/conversations/{id}/messages
- Update Communication.tsx: use convId for AI streaming, remove aiSessionId, use fetchAiMessages for AI conversations
- Update AiChatPanel.tsx: create comm conversation instead of AI session, use new fetchMessages
- Update AISidebar.tsx: remove ChatWindow import, show placeholder

tsc clean, build successful, backend import OK

frontend/src/routes/index.tsx

@@ -38,12 +38,10 @@ const TrashPage = React.lazy(() => import('@/pages/Trash').then(m => ({ default:
 const MailPage = React.lazy(() => import('@/pages/Mail').then(m => ({ default: m.MailPage })));
 const MailSettingsPage = React.lazy(() => import('@/pages/MailSettings').then(m => ({ default: m.MailSettingsPage })));
 const SettingsNotificationsPage = React.lazy(() => import('@/pages/SettingsNotifications').then(m => ({ default: m.SettingsNotificationsPage })));
-const AIAssistantPage = React.lazy(() => import('@/pages/AIAssistant').then(m => ({ default: m.AIAssistantPage })));
 const AISettingsPage = React.lazy(() => import('@/pages/AISettings').then(m => ({ default: m.AISettingsPage })));
 const ProactiveAISettings = React.lazy(() => import('@/pages/ProactiveAISettings').then(m => ({ default: m.ProactiveAISettings })));
 const SettingsThemePage = React.lazy(() => import('@/pages/SettingsTheme').then(m => ({ default: m.SettingsThemePage })));
 const SettingsMcpPage = React.lazy(() => import('@/pages/SettingsMcp').then(m => ({ default: m.SettingsMcpPage })));
-const AIAssistantStandalonePage = React.lazy(() => import('@/pages/AIAssistantStandalone').then(m => ({ default: m.AIAssistantStandalonePage })));
 const DmsStandalonePage = React.lazy(() => import('@/pages/DmsStandalone').then(m => ({ default: m.DmsStandalonePage })));
 const CalendarStandalonePage = React.lazy(() => import('@/pages/CalendarStandalone').then(m => ({ default: m.CalendarStandalonePage })));
 const MailStandalonePage = React.lazy(() => import('@/pages/MailStandalone').then(m => ({ default: m.MailStandalonePage })));
@@ -111,8 +109,6 @@ const router = createBrowserRouter([
     element: <LoginPage />,
   },
   {
-    path: '/ai-assistant-standalone',
-    element: <ErrorBoundary>{withSuspense(<AIAssistantStandalonePage />)}</ErrorBoundary>,
   },
   {
     path: '/dms-standalone',
@@ -178,7 +174,6 @@ const router = createBrowserRouter([
           { path: 'overview', element: withSuspense(<AgentsOverviewPage />) },
           { path: 'automation', element: withSuspense(<AutomationDashboardPage />) },
           { path: 'automation-settings', element: withSuspense(<AutomationSettingsPage />) },
-          { path: 'ai-assistant', element: withSuspense(<AIAssistantPage />) },
           { path: 'ai-settings', element: withSuspense(<AISettingsPage />) },
           { path: 'ai-proactive', element: withSuspense(<ProactiveAISettings />) },
           { path: '*', element: withSuspense(<AgentsPlaceholderPage />) },
@@ -258,7 +253,6 @@ const router = createBrowserRouter([
       { path: '/trash', element: <PermissionRoute permission="contacts:read">{withSuspense(<TrashPage />)}</PermissionRoute> },
       { path: '/mail', element: <PermissionRoute permission="mail:read">{withSuspense(<MailPage />)}</PermissionRoute> },
       { path: '/mail/settings', element: <PermissionRoute permission="mail:read">{withSuspense(<MailSettingsPage />)}</PermissionRoute> },
-      { path: '/ai-assistant', element: <PermissionRoute permission="ai:read">{withSuspense(<AIAssistantPage />)}</PermissionRoute> },
       { path: '/reports', element: <PermissionRoute permission="reports:read">{withSuspense(<ReportsPage />)}</PermissionRoute> },
       { path: '/tasks', element: <PermissionRoute permission="tasks:read">{withSuspense(<TasksPage />)}</PermissionRoute> },
       { path: '/communication', element: <PermissionRoute permission="communication:read">{withSuspense(<CommunicationPage />)}</PermissionRoute> },