feat(admin): enhance adult verification file handling and localization

- Added a new method in AdminService to resolve adult verification file paths, improving file retrieval logic.
- Updated the AdminVerificationView to display a message when the verification document is missing.
- Localized the missing document message in German, English, and Spanish for better user experience.

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

@@ -45,6 +45,7 @@
             "previewTitle": "Nachweis-Vorschau",
             "closePreview": "Vorschau schließen",
             "previewUnavailable": "Für diesen Dateityp ist hier keine Vorschau verfügbar.",
+            "documentMissing": "Die Nachweisdatei wurde auf dem Server nicht gefunden.",
             "empty": "Keine passenden Anfragen gefunden.",
             "loadError": "Die Freigaben konnten nicht geladen werden.",
             "updateError": "Der Status konnte nicht geändert werden.",

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

@@ -45,6 +45,7 @@
             "previewTitle": "Proof preview",
             "closePreview": "Close preview",
             "previewUnavailable": "No inline preview is available for this file type.",
+            "documentMissing": "The verification file was not found on the server.",
             "empty": "No matching requests found.",
             "loadError": "Could not load approvals.",
             "updateError": "Could not update the status.",

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

@@ -45,6 +45,7 @@
             "previewTitle": "Vista previa de la prueba",
             "closePreview": "Cerrar vista previa",
             "previewUnavailable": "No hay vista previa integrada para este tipo de archivo.",
+            "documentMissing": "El archivo de verificación no se encontró en el servidor.",
             "empty": "No se han encontrado solicitudes.",
             "loadError": "No se pudieron cargar las aprobaciones.",
             "updateError": "No se pudo actualizar el estado.",