perf: Fix all 7 code analysis issues

HIGH (Performance):
- Replace 8 sync file operations with aiofiles in async context (storage, mail,
  report_generator, dms_bridge, ai_assistant)
- Frontend bundle splitting: manualChunks for react-vendor, ui-components, tanstack,
  markdown, icons, utils, i18n (ui chunk 936K → ~19K)

MEDIUM (Architecture):
- Worker circular deps: Replace direct plugin imports with job_registry.py pattern
  (register_job/get_all_jobs, importlib-based lazy loading)
- App-wide ErrorBoundary: New ErrorBoundary.tsx component, wrapped in AppShell
  and all standalone routes

LOW (Code Quality):
- N+1 query fix: selectinload(Contact.contact_persons) in list_contacts()
- O(n²) dedup fix: SQL GROUP BY for email/phone duplicates, Dict-based name grouping
- Response format standardization: 7 routes converted from plain arrays to
  {items: [...], total: N} format

frontend/src/components/ErrorBoundary.tsx

@@ -0,0 +1,82 @@
+import React from 'react';
+
+interface ErrorBoundaryProps {
+  children: React.ReactNode;
+  fallback?: React.ReactNode;
+}
+
+interface ErrorBoundaryState {
+  hasError: boolean;
+  error: Error | null;
+}
+
+/**
+ * App-wide ErrorBoundary that catches React rendering errors
+ * and displays a fallback UI with a reload button.
+ */
+export class ErrorBoundary extends React.Component<ErrorBoundaryProps, ErrorBoundaryState> {
+  constructor(props: ErrorBoundaryProps) {
+    super(props);
+    this.state = { hasError: false, error: null };
+  }
+
+  static getDerivedStateFromError(error: Error): ErrorBoundaryState {
+    return { hasError: true, error };
+  }
+
+  componentDidCatch(error: Error, errorInfo: React.ErrorInfo): void {
+    console.error('[ErrorBoundary] Uncaught error:', error, errorInfo);
+  }
+
+  handleReload = (): void => {
+    window.location.reload();
+  };
+
+  render(): React.ReactNode {
+    if (this.state.hasError) {
+      if (this.props.fallback) {
+        return this.props.fallback;
+      }
+
+      return (
+        <div
+          className="flex items-center justify-center min-h-screen bg-secondary-50 p-8"
+          role="alert"
+        >
+          <div className="max-w-md w-full bg-white rounded-lg shadow-lg p-8 text-center">
+            <div className="text-red-500 text-6xl mb-4" aria-hidden="true">
+              ⚠️
+            </div>
+            <h1 className="text-2xl font-bold text-secondary-900 mb-2">
+              Ein Fehler ist aufgetreten
+            </h1>
+            <p className="text-secondary-600 mb-6">
+              Die Anwendung konnte nicht geladen werden. Bitte versuchen Sie es erneut.
+            </p>
+            {this.state.error && (
+              <details className="mb-6 text-left">
+                <summary className="cursor-pointer text-sm text-secondary-500 hover:text-secondary-700">
+                  Fehlerdetails
+                </summary>
+                <pre className="mt-2 p-3 bg-secondary-100 rounded text-xs text-secondary-700 overflow-auto max-h-32">
+                  {this.state.error.message}
+                  {this.state.error.stack && `\n\n${this.state.error.stack}`}
+                </pre>
+              </details>
+            )}
+            <button
+              onClick={this.handleReload}
+              className="inline-flex items-center px-6 py-3 bg-primary-600 text-white font-medium rounded-lg hover:bg-primary-700 focus:outline-none focus:ring-2 focus:ring-primary-500 focus:ring-offset-2 transition-colors"
+            >
+              Neu laden
+            </button>
+          </div>
+        </div>
+      );
+    }
+
+    return this.props.children;
+  }
+}
+
+export default ErrorBoundary;

frontend/src/components/layout/AppShell.tsx

@@ -10,6 +10,7 @@ import { useAIUIControl } from '@/hooks/useAIUIControl';
 import { PluginRegistry } from '@/components/plugins/PluginRegistry';
 import { AIUIControlIndicator } from '@/components/ai-ui-control/AIUIControlIndicator';
 import { WindowContainer } from '@/components/window/WindowContainer';
+import { ErrorBoundary } from '@/components/ErrorBoundary';
 
 export function AppShell() {
   const location = useLocation();
@@ -38,7 +39,9 @@ export function AppShell() {
             id="main-content"
             data-testid="content-area"
           >
-            <Outlet />
+            <ErrorBoundary>
+              <Outlet />
+            </ErrorBoundary>
           </main>
         </div>
       </div>

frontend/src/routes/index.tsx

@@ -7,6 +7,7 @@ import { PasswordResetRequestPage } from '@/pages/PasswordResetRequest';
 import { PasswordResetConfirmPage } from '@/pages/PasswordResetConfirm';
 import { Loader2 } from 'lucide-react';
 import { PluginRouteRenderer } from '@/components/plugins/PluginRouteRenderer';
+import { ErrorBoundary } from '@/components/ErrorBoundary';
 
 // Lazy-loaded pages (code-splitting)
 const DashboardPage = React.lazy(() => import('@/pages/Dashboard').then(m => ({ default: m.DashboardPage })));
@@ -73,23 +74,23 @@ const router = createBrowserRouter([
   },
   {
     path: '/ai-assistant-standalone',
-    element: withSuspense(<AIAssistantStandalonePage />),
+    element: <ErrorBoundary>{withSuspense(<AIAssistantStandalonePage />)}</ErrorBoundary>,
   },
   {
     path: '/dms-standalone',
-    element: withSuspense(<DmsStandalonePage />),
+    element: <ErrorBoundary>{withSuspense(<DmsStandalonePage />)}</ErrorBoundary>,
   },
   {
     path: '/calendar-standalone',
-    element: withSuspense(<CalendarStandalonePage />),
+    element: <ErrorBoundary>{withSuspense(<CalendarStandalonePage />)}</ErrorBoundary>,
   },
   {
     path: '/mail-standalone',
-    element: withSuspense(<MailStandalonePage />),
+    element: <ErrorBoundary>{withSuspense(<MailStandalonePage />)}</ErrorBoundary>,
   },
   {
     path: '/contacts-standalone',
-    element: withSuspense(<ContactsStandalonePage />),
+    element: <ErrorBoundary>{withSuspense(<ContactsStandalonePage />)}</ErrorBoundary>,
   },
   {
     path: '/password-reset',