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/en/admin.json

@@ -42,6 +42,9 @@
             "reject": "Reject",
             "resetPending": "Set pending",
             "openDocument": "Open document",
+            "previewTitle": "Proof preview",
+            "closePreview": "Close preview",
+            "previewUnavailable": "No inline preview is available for this file type.",
             "empty": "No matching requests found.",
             "loadError": "Could not load approvals.",
             "updateError": "Could not update the status.",