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/Compat/NavigationRegistry.php

@@ -13,7 +13,7 @@ if ( ! defined('ABSPATH') ) exit;
  */
 class NavigationRegistry {
     const NAV_OPTION = 'wnw_nav_tree';
-    const NAV_VERSION = '1.0.7'; // Removed 'New Coupon' from submenu
+    const NAV_VERSION = '1.0.8'; // Added Modules to Settings menu
     
     /**
      * Initialize hooks
@@ -105,7 +105,7 @@ class NavigationRegistry {
      * @return array Base navigation tree
      */
     private static function get_base_tree(): array {
-        return [
+        $tree = [
             [
                 'key'      => 'dashboard',
                 'label'    => __('Dashboard', 'woonoow'),
@@ -160,10 +160,7 @@ class NavigationRegistry {
                 'label'    => __('Marketing', 'woonoow'),
                 'path'     => '/marketing',
                 'icon'     => 'mail',
-                'children' => [
-                    ['label' => __('Newsletter', 'woonoow'), 'mode' => 'spa', 'path' => '/marketing/newsletter'],
-                    ['label' => __('Coupons', 'woonoow'), 'mode' => 'spa', 'path' => '/coupons'],
-                ],
+                'children' => self::get_marketing_children(),
             ],
             [
                 'key'      => 'appearance',
@@ -190,6 +187,27 @@ class NavigationRegistry {
                 'children' => self::get_settings_children(),
             ],
         ];
+        
+        return $tree;
+    }
+    
+    /**
+     * Get marketing submenu children
+     * 
+     * @return array Marketing submenu items
+     */
+    private static function get_marketing_children(): array {
+        $children = [];
+        
+        // Newsletter - only if module enabled
+        if (\WooNooW\Core\ModuleRegistry::is_enabled('newsletter')) {
+            $children[] = ['label' => __('Newsletter', 'woonoow'), 'mode' => 'spa', 'path' => '/marketing/newsletter'];
+        }
+        
+        // Coupons - always available
+        $children[] = ['label' => __('Coupons', 'woonoow'), 'mode' => 'spa', 'path' => '/coupons'];
+        
+        return $children;
     }
     
     /**
@@ -208,6 +226,7 @@ class NavigationRegistry {
             ['label' => __('Tax', 'woonoow'), 'mode' => 'spa', 'path' => '/settings/tax'],
             ['label' => __('Customers', 'woonoow'), 'mode' => 'spa', 'path' => '/settings/customers'],
             ['label' => __('Notifications', 'woonoow'), 'mode' => 'spa', 'path' => '/settings/notifications'],
+            ['label' => __('Modules', 'woonoow'), 'mode' => 'spa', 'path' => '/settings/modules'],
             ['label' => __('Developer', 'woonoow'), 'mode' => 'spa', 'path' => '/settings/developer'],
         ];