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/i18n/locales/de.json

@@ -1440,5 +1440,31 @@
     "targetRoom": "Ziel-Raum",
     "targetRoomDescription": "Name des Raums in der Kommunikation, an den Status-Meldungen gesendet werden.",
     "defaultRoomName": "Live KI"
+  },
+  "importexport": {
+    "dialogTitle": "Import / Export",
+    "export": "Export",
+    "import": "Import",
+    "download": "Herunterladen",
+    "exportHint": "Daten des Moduls {{entity}} exportieren — Format wählen:",
+    "chooseFile": "Datei wählen (CSV, JSON oder XLSX)",
+    "step1": "Datei",
+    "step2": "Zuordnung",
+    "step3": "Prüfung",
+    "step4": "Ergebnis",
+    "mappingHint": "{{total}} Zeilen erkannt — Dateispalten den Zielfeldern zuordnen:",
+    "fileColumn": "Dateispalte",
+    "targetField": "Zielfeld",
+    "skip": "überspringen",
+    "validate": "Prüfen",
+    "valid": "gültig",
+    "invalid": "ungültig",
+    "row": "Zeile",
+    "executeImport": "Import ausführen",
+    "total": "gesamt",
+    "imported": "importiert",
+    "failed": "fehlgeschlagen",
+    "importDone": "Import abgeschlossen",
+    "backgroundRunning": "Import läuft im Hintergrund …"
   }
 }

frontend/src/i18n/locales/en.json

@@ -1440,5 +1440,31 @@
     "targetRoom": "Target room",
     "targetRoomDescription": "Name of the room in Communication that status messages are sent to.",
     "defaultRoomName": "Live AI"
+  },
+  "importexport": {
+    "dialogTitle": "Import / Export",
+    "export": "Export",
+    "import": "Import",
+    "download": "Download",
+    "exportHint": "Export data of module {{entity}} — choose format:",
+    "chooseFile": "Choose file (CSV, JSON or XLSX)",
+    "step1": "File",
+    "step2": "Mapping",
+    "step3": "Check",
+    "step4": "Result",
+    "mappingHint": "{{total}} rows detected — map file columns to target fields:",
+    "fileColumn": "File column",
+    "targetField": "Target field",
+    "skip": "skip",
+    "validate": "Check",
+    "valid": "valid",
+    "invalid": "invalid",
+    "row": "Row",
+    "executeImport": "Run import",
+    "total": "total",
+    "imported": "imported",
+    "failed": "failed",
+    "importDone": "Import finished",
+    "backgroundRunning": "Import running in background …"
   }
 }