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/lib/utils.ts

@@ -0,0 +1,54 @@
+import { type ClassValue, clsx } from "clsx"
+import { twMerge } from "tailwind-merge"
+
+/**
+ * Merge Tailwind classes with clsx
+ */
+export function cn(...inputs: ClassValue[]) {
+  return twMerge(clsx(inputs))
+}
+
+/**
+ * Format price
+ */
+export function formatPrice(price: number | string, currency: string = 'USD'): string {
+  const numPrice = typeof price === 'string' ? parseFloat(price) : price;
+  
+  return new Intl.NumberFormat('en-US', {
+    style: 'currency',
+    currency: currency,
+  }).format(numPrice);
+}
+
+/**
+ * Format date
+ */
+export function formatDate(date: string | Date): string {
+  const dateObj = typeof date === 'string' ? new Date(date) : date;
+  
+  return new Intl.DateTimeFormat('en-US', {
+    year: 'numeric',
+    month: 'long',
+    day: 'numeric',
+  }).format(dateObj);
+}
+
+/**
+ * Debounce function
+ */
+export function debounce<T extends (...args: any[]) => any>(
+  func: T,
+  wait: number
+): (...args: Parameters<T>) => void {
+  let timeout: ReturnType<typeof setTimeout>;
+  
+  return function executedFunction(...args: Parameters<T>) {
+    const later = () => {
+      clearTimeout(timeout);
+      func(...args);
+    };
+    
+    clearTimeout(timeout);
+    timeout = setTimeout(later, wait);
+  };
+}