feat(#359): W4a Phase 2 — zentraler Import/Export-Dialog (Frontend)

- ImportExportDialog.tsx (neu): Modal lg/xl nach bestehendem ui/Modal-Muster
- Export-Tab: Formatauswahl (csv/xlsx/json), Download, Fehler-Handling
- Import-Tab: 4 Schritte (Datei -> Mapping -> Dry-Run -> Ausführung+Report),
  Mapping-Vorschau mit Modul-Heuristik, Background-Job-Polling ab 1000 Zeilen
- i18n: 24 importexport.*-Keys in de.json + en.json (keine hardcoded Strings)
- Integration: ContactsList Toolbar-Button (contacts:read-Gate, Upload-Icon,
  entityType=contacts vorgewählt) über bestehendes pluginToolbarStore-Muster

Gates: Vitest 12/12 (routePermissions + importExportDialog), tsc exit 0,
Production-Build exit 0 (vor Commit). 6 Failures in contacts/shell Suiten
als Vorbestand bewiesen (Stash-Test: identisch auf clean HEAD f27f047).

fixes #359 (Phase 2)

frontend/src/pages/ContactsList.tsx

@@ -19,7 +19,8 @@ import { SavedFilters } from '@/components/SavedFilters';
 import { SavedFilterBar } from '@/components/common/SavedFilterBar';
 import { TagSelector } from '@/components/tags/TagSelector';
 import type { Tag } from '@/api/tags';
-import { ArrowDownAZ, ArrowUpZA, Bookmark, ChevronLeft, ExternalLink, LayoutGrid, List, Plus, Printer, Table2, X } from 'lucide-react';
+import { ArrowDownAZ, ArrowUpZA, Bookmark, ChevronLeft, ExternalLink, LayoutGrid, List, Plus, Printer, Table2, Upload, X } from 'lucide-react';
+import { ImportExportDialog } from '@/components/importexport/ImportExportDialog';
 import { FilterPanel, applyFilters, emptyFilterState, type FilterState, type SavedFilter } from '@/components/contacts/FilterPanel';
 import { SortPanel, applySorting, emptySortState, type SortState } from '@/components/contacts/SortPanel';
 import { GroupPanel, applyGrouping, emptyGroupState, type GroupState, type GroupedContacts } from '@/components/contacts/GroupPanel';
@@ -222,6 +223,9 @@ export function ContactsListPage() {
     setActiveView('detail');
   }, []);
 
+  // Import/Export dialog (W4a #359)
+  const [importExportOpen, setImportExportOpen] = useState(false);
+
   // Handle create
   const handleCreate = useCallback(() => {
     const windowId = openWindow({
@@ -374,6 +378,16 @@ export function ContactsListPage() {
         icon: <Plus className="w-3.5 h-3.5" strokeWidth={2} />,
         onClick: handleCreate,
       }] : []),
+      // Import/Export dialog (W4a #359)
+      ...(canAccess('contacts:read') ? [{
+        id: 'import-export',
+        plugin: 'contacts',
+        label: t('importexport.dialogTitle'),
+        type: 'button' as const,
+        group: 'create',
+        icon: <Upload className="w-3.5 h-3.5" strokeWidth={2} />,
+        onClick: () => setImportExportOpen(true),
+      }] : []),
       // View mode dropdown (list / table / cards)
       {
         id: 'view-mode',
@@ -782,6 +796,13 @@ export function ContactsListPage() {
         hasSort={sortState.conditions.length > 0}
         hasFolder={selectedFilter !== 'all' || multiSelectFolders.length > 0}
       />
+
+      {/* Import/Export Dialog (W4a #359) */}
+      <ImportExportDialog
+        open={importExportOpen}
+        onClose={() => setImportExportOpen(false)}
+        entityType="contacts"
+      />
     </div>
   );
 }