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

admin-spa/src/lib/wp-media.ts

@@ -163,3 +163,52 @@ export function openWPMediaFavicon(onSelect: (file: WPMediaFile) => void): void
     onSelect
   );
 }
+
+/**
+ * Open WordPress Media Modal for Multiple Images (Product Gallery)
+ */
+export function openWPMediaGallery(onSelect: (files: WPMediaFile[]) => void): void {
+  // Check if WordPress media is available
+  if (typeof window.wp === 'undefined' || typeof window.wp.media === 'undefined') {
+    console.error('WordPress media library is not available');
+    alert('WordPress Media library is not loaded.');
+    return;
+  }
+
+  // Create media frame with multiple selection
+  const frame = window.wp.media({
+    title: 'Select or Upload Product Images',
+    button: {
+      text: 'Add to Gallery',
+    },
+    multiple: true,
+    library: {
+      type: 'image',
+    },
+  });
+
+  // Handle selection
+  frame.on('select', () => {
+    const selection = frame.state().get('selection') as any;
+    const files: WPMediaFile[] = [];
+    
+    selection.map((attachment: any) => {
+      const data = attachment.toJSON();
+      files.push({
+        url: data.url,
+        id: data.id,
+        title: data.title || data.filename,
+        filename: data.filename,
+        alt: data.alt || '',
+        width: data.width,
+        height: data.height,
+      });
+      return attachment;
+    });
+
+    onSelect(files);
+  });
+
+  // Open modal
+  frame.open();
+}