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

admin-spa/src/routes/Marketing/Newsletter.tsx

@@ -8,6 +8,7 @@ import { Download, Trash2, Mail, Search } from 'lucide-react';
 import { toast } from 'sonner';
 import { api } from '@/lib/api';
 import { useNavigate } from 'react-router-dom';
+import { useModules } from '@/hooks/useModules';
 import {
   Table,
   TableBody,
@@ -21,6 +22,27 @@ export default function NewsletterSubscribers() {
   const [searchQuery, setSearchQuery] = useState('');
   const queryClient = useQueryClient();
   const navigate = useNavigate();
+  const { isEnabled } = useModules();
+
+  if (!isEnabled('newsletter')) {
+    return (
+      <SettingsLayout
+        title="Newsletter Subscribers"
+        description="Newsletter module is disabled"
+      >
+        <div className="bg-yellow-50 dark:bg-yellow-950/20 border border-yellow-200 dark:border-yellow-900 rounded-lg p-6 text-center">
+          <Mail className="h-12 w-12 text-yellow-600 mx-auto mb-3" />
+          <h3 className="font-semibold text-lg mb-2">Newsletter Module Disabled</h3>
+          <p className="text-sm text-muted-foreground mb-4">
+            The newsletter module is currently disabled. Enable it in Settings &gt; Modules to use this feature.
+          </p>
+          <Button onClick={() => navigate('/settings/modules')}>
+            Go to Module Settings
+          </Button>
+        </div>
+      </SettingsLayout>
+    );
+  }
 
   const { data: subscribersData, isLoading } = useQuery({
     queryKey: ['newsletter-subscribers'],