feat: Implement centralized module management system

- Add ModuleRegistry for managing built-in modules (newsletter, wishlist, affiliate, subscription, licensing)
- Add ModulesController REST API for module enable/disable
- Create Modules settings page with category grouping and toggle controls
- Integrate module checks across admin-spa and customer-spa
- Add useModules hook for both SPAs to check module status
- Hide newsletter from footer builder when module disabled
- Hide wishlist features when module disabled (product cards, account menu, wishlist page)
- Protect wishlist API endpoints with module checks
- Auto-update navigation tree when modules toggled
- Clean up obsolete documentation files
- Add comprehensive documentation:
  - MODULE_SYSTEM_IMPLEMENTATION.md
  - MODULE_INTEGRATION_SUMMARY.md
  - ADDON_MODULE_INTEGRATION.md (proposal)
  - ADDON_MODULE_DESIGN_DECISIONS.md (design doc)
  - FEATURE_ROADMAP.md
  - SHIPPING_INTEGRATION.md

Module system provides:
- Centralized enable/disable for all features
- Automatic navigation updates
- Frontend/backend integration
- Foundation for addon-module unification

includes/Frontend/WishlistController.php

@@ -4,6 +4,7 @@ namespace WooNooW\Frontend;
 use WP_REST_Request;
 use WP_REST_Response;
 use WP_Error;
+use WooNooW\Core\ModuleRegistry;
 
 class WishlistController {
     
@@ -60,6 +61,10 @@ class WishlistController {
      * Get wishlist items with product details
      */
     public static function get_wishlist(WP_REST_Request $request) {
+        if (!ModuleRegistry::is_enabled('wishlist')) {
+            return new WP_Error('module_disabled', __('Wishlist module is disabled', 'woonoow'), ['status' => 403]);
+        }
+        
         $user_id = get_current_user_id();
         $wishlist = get_user_meta($user_id, 'woonoow_wishlist', true);
         
@@ -98,6 +103,10 @@ class WishlistController {
      * Add product to wishlist
      */
     public static function add_to_wishlist(WP_REST_Request $request) {
+        if (!ModuleRegistry::is_enabled('wishlist')) {
+            return new WP_Error('module_disabled', __('Wishlist module is disabled', 'woonoow'), ['status' => 403]);
+        }
+        
         $user_id = get_current_user_id();
         $product_id = $request->get_param('product_id');
         
@@ -137,6 +146,10 @@ class WishlistController {
      * Remove product from wishlist
      */
     public static function remove_from_wishlist(WP_REST_Request $request) {
+        if (!ModuleRegistry::is_enabled('wishlist')) {
+            return new WP_Error('module_disabled', __('Wishlist module is disabled', 'woonoow'), ['status' => 403]);
+        }
+        
         $user_id = get_current_user_id();
         $product_id = (int) $request->get_param('product_id');
         
@@ -165,6 +178,10 @@ class WishlistController {
      * Clear entire wishlist
      */
     public static function clear_wishlist(WP_REST_Request $request) {
+        if (!ModuleRegistry::is_enabled('wishlist')) {
+            return new WP_Error('module_disabled', __('Wishlist module is disabled', 'woonoow'), ['status' => 403]);
+        }
+        
         $user_id = get_current_user_id();
         delete_user_meta($user_id, 'woonoow_wishlist');