added mobile category extension

models/category.ts

@@ -9,4 +9,8 @@ export interface Category { //_id is not needed as it is created by default
 
 export interface MongoCategory extends Omit<Category, 'shelves'> {
     shelves?: Array<string>, // Shelf IDs
+}
+
+export interface MobileCategory extends MongoCategory {
+    _id: string,
 }