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/hooks/useModules.ts

@@ -0,0 +1,31 @@
+import { useQuery } from '@tanstack/react-query';
+import { api } from '@/lib/api/client';
+
+interface ModulesResponse {
+  enabled: string[];
+}
+
+/**
+ * Hook to check if modules are enabled
+ * Uses public endpoint, cached for performance
+ */
+export function useModules() {
+  const { data, isLoading } = useQuery<ModulesResponse>({
+    queryKey: ['modules-enabled'],
+    queryFn: async () => {
+      const response = await api.get('/modules/enabled') as any;
+      return response.data;
+    },
+    staleTime: 5 * 60 * 1000, // Cache for 5 minutes
+  });
+
+  const isEnabled = (moduleId: string): boolean => {
+    return data?.enabled?.includes(moduleId) ?? false;
+  };
+
+  return {
+    enabledModules: data?.enabled ?? [],
+    isEnabled,
+    isLoading,
+  };
+}