feat(admin): add document preview functionality to adult verification

- Implemented a preview section for adult verification documents, allowing users to view images and PDFs inline.
- Added localization support for preview titles and messages in German, English, and Spanish.
- Enhanced the component's state management to handle preview visibility and cleanup.

frontend/src/i18n/locales/es/admin.json

@@ -42,6 +42,9 @@
             "reject": "Rechazar",
             "resetPending": "Poner en revisión",
             "openDocument": "Abrir documento",
+            "previewTitle": "Vista previa de la prueba",
+            "closePreview": "Cerrar vista previa",
+            "previewUnavailable": "No hay vista previa integrada para este tipo de archivo.",
             "empty": "No se han encontrado solicitudes.",
             "loadError": "No se pudieron cargar las aprobaciones.",
             "updateError": "No se pudo actualizar el estado.",