Localize label for activity hidden fields

Replaced the hardcoded 'Activity hidden fields' label in EditActivityModalComponent.vue with a localized string. Added the corresponding translation key to the i18n JSON file to support localization.

frontend/app/src/components/Activities/Modals/EditActivityModalComponent.vue

@@ -256,7 +256,7 @@
               <option :value="false">{{ $t('generalItems.no') }}</option>
             </select>
             <div class="d-flex justify-content-start align-items-center">
-              <span>Activity hidden fields</span>
+              <span>{{ $t('editActivityModalComponent.modalEditActivityHiddenItemsLabel') }}</span>
               <!-- button toggle hidden fields -->
               <a
                 class="btn btn-link btn-lg link-body-emphasis"

frontend/app/src/i18n/us/components/activities/modals/editActivityModalComponent.json

@@ -57,6 +57,7 @@
   "modalEditActivityVisibilityOption1": "Followers",
   "modalEditActivityVisibilityOption2": "Private",
   "modalEditActivityIsHiddenLabel": "Is hidden",
+  "modalEditActivityHiddenItemsLabel": "Activity hidden fields",
   "modalEditActivityHideStartTimeLabel": "Hide start time",
   "modalEditActivityHideLocationLabel": "Hide location",
   "modalEditActivityHideMapLabel": "Hide map",