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/Api/Routes.php

@@ -21,6 +21,7 @@ use WooNooW\Api\ProductsController;
 use WooNooW\Api\CouponsController;
 use WooNooW\Api\CustomersController;
 use WooNooW\Api\NewsletterController;
+use WooNooW\Api\ModulesController;
 use WooNooW\Frontend\ShopController;
 use WooNooW\Frontend\CartController as FrontendCartController;
 use WooNooW\Frontend\AccountController;
@@ -123,6 +124,10 @@ class Routes {
             // Newsletter controller
             NewsletterController::register_routes();
             
+            // Modules controller
+            $modules_controller = new ModulesController();
+            $modules_controller->register_routes();
+            
             // Frontend controllers (customer-facing)
             ShopController::register_routes();
             FrontendCartController::register_routes();