feat: Add product images support with WP Media Library integration

- Add WP Media Library integration for product and variation images
- Support images array (URLs) conversion to attachment IDs
- Add images array to API responses (Admin & Customer SPA)
- Implement drag-and-drop sortable images in Admin product form
- Add image gallery thumbnails in Customer SPA product page
- Initialize WooCommerce session for guest cart operations
- Fix product variations and attributes display in Customer SPA
- Add variation image field in Admin SPA

Changes:
- includes/Api/ProductsController.php: Handle images array, add to responses
- includes/Frontend/ShopController.php: Add images array for customer SPA
- includes/Frontend/CartController.php: Initialize WC session for guests
- admin-spa/src/lib/wp-media.ts: Add openWPMediaGallery function
- admin-spa/src/routes/Products/partials/tabs/GeneralTab.tsx: WP Media + sortable images
- admin-spa/src/routes/Products/partials/tabs/VariationsTab.tsx: Add variation image field
- customer-spa/src/pages/Product/index.tsx: Add gallery thumbnails display

customer-spa/src/types/product.ts

@@ -0,0 +1,45 @@
+/**
+ * Product Types
+ */
+
+export interface ProductCategory {
+  id: number;
+  name: string;
+  slug: string;
+  count?: number;
+}
+
+export interface Product {
+  id: number;
+  name: string;
+  slug: string;
+  type: string;
+  status: string;
+  price: string;
+  regular_price?: string;
+  sale_price?: string;
+  on_sale: boolean;
+  stock_status: 'instock' | 'outofstock' | 'onbackorder';
+  stock_quantity?: number;
+  image?: string;
+  images?: string[];
+  short_description?: string;
+  description?: string;
+  sku?: string;
+  categories?: ProductCategory[];
+  tags?: any[];
+  attributes?: any[];
+  variations?: number[];
+  permalink?: string;
+}
+
+export interface ProductsResponse {
+  products: Product[];
+  total: number;
+  total_pages: number;
+  current_page: number;
+}
+
+export interface CategoriesResponse {
+  categories: ProductCategory[];
+}