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/views/admin/AdultVerificationView.vue

@@ -44,7 +44,15 @@
               <template v-if="row.adultVerificationRequest">
                 <strong>{{ row.adultVerificationRequest.originalName }}</strong>
                 <span v-if="row.adultVerificationRequest.note">{{ row.adultVerificationRequest.note }}</span>
-                <button type="button" class="secondary" @click="openDocument(row)">
+                <span v-if="!row.adultVerificationDocumentAvailable" class="adult-verification__missing-file">
+                  {{ $t('admin.adultVerification.documentMissing') }}
+                </span>
+                <button
+                  type="button"
+                  class="secondary"
+                  :disabled="!row.adultVerificationDocumentAvailable"
+                  @click="openDocument(row)"
+                >
                   {{ $t('admin.adultVerification.openDocument') }}
                 </button>
               </template>
@@ -149,6 +157,10 @@ export default {
       }
     },
     async openDocument(row) {
+      if (!row.adultVerificationDocumentAvailable) {
+        showApiError(this, null, this.$t('admin.adultVerification.documentMissing'));
+        return;
+      }
       try {
         const response = await apiClient.get(`/api/admin/users/${row.id}/adult-verification/document`, {
           responseType: 'blob'
@@ -269,6 +281,11 @@ export default {
   gap: 4px;
 }
 
+.adult-verification__missing-file {
+  color: #a94442;
+  font-size: 0.9rem;
+}
+
 .adult-verification__actions .secondary {
   background: transparent;
 }