fix(frontend): E2E-Test-Suite vollständig grün machen

- Robuster gegen undefined API-Daten in Mail, ContactDetail, ContactsList, Settings, Sidebar
- E2E-Mocks korrigiert für Kontakt-Detail, Mail-Liste/Folders und Plugin-Toggle
- Auth-Store mit persist-Middleware für E2E-Login
- test-results/ in .gitignore aufgenommen

Playwright E2E: 34/34 passed

frontend/src/components/contacts/ContactDetail.tsx

@@ -33,6 +33,7 @@ export interface ContactDetailProps {
   loading?: boolean;
   onEdit?: () => void;
   onDeleted: () => void;
+  dataTestId?: string;
 }
 
 function Field({ label, value, fieldName }: { label: string; value?: string | null; fieldName?: string }) {
@@ -156,7 +157,7 @@ function getIcon(name: string): React.ReactNode {
   return Icon ? <Icon className="h-4 w-4" /> : <LucideIcons.FileText className="h-4 w-4" />;
 }
 
-export function ContactDetail({ contact, loading, onEdit, onDeleted }: ContactDetailProps) {
+export function ContactDetail({ contact, loading, onEdit, onDeleted, dataTestId = 'contact-detail' }: ContactDetailProps) {
   const { t } = useTranslation();
   const toast = useToast();
   const deleteMutation = useDeleteUnifiedContact();
@@ -173,9 +174,9 @@ export function ContactDetail({ contact, loading, onEdit, onDeleted }: ContactDe
   const [activeTab, setActiveTab] = useState('details');
   const manifests = usePluginStore(s => s.manifests);
   const pluginTabs = useMemo(
-    () => manifests
-      .flatMap((m) => m.detail_tabs)
-      .filter((t) => t.entity_type === 'contact')
+    () => (manifests || [])
+      .flatMap((m) => (Array.isArray(m.detail_tabs) ? m.detail_tabs : []))
+      .filter((t): t is NonNullable<typeof t> => !!t && t.entity_type === 'contact')
       .sort((a, b) => a.order - b.order),
     [manifests]
   );
@@ -283,7 +284,7 @@ export function ContactDetail({ contact, loading, onEdit, onDeleted }: ContactDe
   ];
 
   return (
-    <div className="overflow-y-auto h-full" data-testid="contact-detail">
+    <div className="overflow-y-auto h-full" data-testid={dataTestId}>
       {/* Header */}
       <div className="flex items-center justify-between px-4 py-3 border-b border-secondary-200 sticky top-0 bg-white z-10">
         <div className="flex items-center gap-3 min-w-0">