fix: translate custom provider layer

backend/tests/test_sprint193_end_user_workbench.py

@@ -72,3 +72,4 @@ def test_visible_ai_and_quality_labels_are_end_user_facing() -> None:
     assert "nog niet uitgevoerd" in quality
     assert "Nog geen bestand gekozen." in export_preview
     assert "providerLayerLabel" in providers
+    assert "custom: 'eigen laag'" in providers

frontend/src/components/providers/ProviderPanel.tsx

@@ -31,6 +31,7 @@ function providerLayerLabel(value: string): string {
     parcels: 'percelen',
     water: 'water',
     landuse: 'landgebruik',
+    custom: 'eigen laag',
   }
   return labels[value] ?? value
 }