add Categories management: create CategoriesList component, update admin navbar, and integrate category handling in product forms

client/src/app/interfaces/category.ts

@@ -0,0 +1,4 @@
+export interface Category {
+  id: string | number;
+  name: string;
+}

client/src/app/interfaces/condition.ts

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+export interface Condition {
+  id: string | number;
+  name: string;
+  displayName: string;
+}

client/src/app/interfaces/product.ts

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+import {Category} from './category';
+import {Platform} from './platform';
+import {Condition} from './condition';
+
+export interface Product {
+  id: string | number;
+  title: string;
+  description: string;
+  price: number;
+  quantity: number;
+  complete: boolean;
+  manualIncluded: boolean;
+  category: Category | undefined;
+  platform: Platform | undefined;
+  condition: Condition | undefined;
+}