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

customer-spa/src/pages/Product/index.tsx

@@ -5,6 +5,7 @@ import { apiClient } from '@/lib/api/client';
 import { useCartStore } from '@/lib/cart/store';
 import { useProductSettings } from '@/hooks/useAppearanceSettings';
 import { useWishlist } from '@/hooks/useWishlist';
+import { useModules } from '@/hooks/useModules';
 import { Button } from '@/components/ui/button';
 import Container from '@/components/Layout/Container';
 import { ProductCard } from '@/components/ProductCard';
@@ -25,6 +26,7 @@ export default function Product() {
   const thumbnailsRef = useRef<HTMLDivElement>(null);
   const { addItem } = useCartStore();
   const { isEnabled: wishlistEnabled, isInWishlist, toggleWishlist, isLoggedIn } = useWishlist();
+  const { isEnabled: isModuleEnabled } = useModules();
 
   // Fetch product details by slug
   const { data: product, isLoading, error } = useQuery<ProductType | null>({
@@ -487,7 +489,7 @@ export default function Product() {
                   <ShoppingCart className="h-5 w-5" />
                   Add to Cart
                 </button>
-                {wishlistEnabled && (
+                {isModuleEnabled('wishlist') && wishlistEnabled && (
                   <button 
                     onClick={() => product && toggleWishlist(product.id)}
                     className={`w-full h-14 flex items-center justify-center gap-2 rounded-xl font-semibold text-base border-2 transition-all ${