DEV: Indicate whether categories are missing on categories page (#27608)

app/assets/javascripts/discourse/app/components/parent-category-row.hbs

@@ -40,6 +40,12 @@
                 @listType={{this.listType}}
               />
             {{/each}}
+            {{#if (gt this.category.unloadedSubcategoryCount 0)}}
+              {{i18n
+                "category_row.subcategory_count"
+                count=this.category.unloadedSubcategoryCount
+              }}
+            {{/if}}
           </tbody>
         </table>
       {{else if this.category.subcategories}}
@@ -50,6 +56,12 @@
               @listType={{this.listType}}
             />
           {{/each}}
+          {{#if (gt this.category.unloadedSubcategoryCount 0)}}
+            {{i18n
+              "category_row.subcategory_count"
+              count=this.category.unloadedSubcategoryCount
+            }}
+          {{/if}}
         </div>
       {{/if}}
     </td>

app/assets/javascripts/discourse/app/models/category.js

@@ -491,6 +491,11 @@ export default class Category extends RestModel {
     return this.site.categories.filterBy("parent_category_id", this.id);
   }
 
+  @computed("subcategories")
+  get unloadedSubcategoryCount() {
+    return this.subcategory_count - this.subcategories.length;
+  }
+
   @computed("subcategory_list")
   get serializedSubcategories() {
     return this.subcategory_list?.map((c) => Category.create(c));