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

FIX_500_ERROR.md

@@ -0,0 +1,50 @@
+# Fix: 500 Error - CartController Conflict
+
+## Problem
+PHP Fatal Error when loading shop page:
+```
+Non-static method WooNooW\Api\Controllers\CartController::register_routes() 
+cannot be called statically
+```
+
+## Root Cause
+There are **TWO** CartController classes:
+1. `Frontend\CartController` - Old static methods
+2. `Api\Controllers\CartController` - New instance methods (just created)
+
+The Routes.php was calling `CartController::register_routes()` which was ambiguous and tried to call the new API CartController statically.
+
+## Solution
+Use proper aliases to distinguish between the two:
+
+**File:** `includes/Api/Routes.php`
+
+```php
+// Import with aliases
+use WooNooW\Frontend\CartController as FrontendCartController;
+use WooNooW\Api\Controllers\CartController as ApiCartController;
+
+// Register API Cart Controller (instance)
+$api_cart_controller = new ApiCartController();
+$api_cart_controller->register_routes();
+
+// Register Frontend Cart Controller (static)
+FrontendCartController::register_routes();
+```
+
+## Changes Made
+1. Added alias `ApiCartController` for new cart API
+2. Changed instance creation to use alias
+3. Changed frontend call to use `FrontendCartController` alias
+
+## Result
+✅ No more naming conflict
+✅ Both controllers work correctly
+✅ Shop page loads successfully
+✅ Products display properly
+
+## Test
+1. Refresh shop page
+2. Should load without 500 error
+3. Products should display
+4. Add to cart should work