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/Appearance/Footer.tsx

@@ -11,6 +11,7 @@ import { Textarea } from '@/components/ui/textarea';
 import { Plus, X } from 'lucide-react';
 import { toast } from 'sonner';
 import { api } from '@/lib/api';
+import { useModules } from '@/hooks/useModules';
 
 interface SocialLink {
   id: string;
@@ -36,6 +37,7 @@ interface ContactData {
 }
 
 export default function AppearanceFooter() {
+  const { isEnabled } = useModules();
   const [loading, setLoading] = useState(true);
   const [columns, setColumns] = useState('4');
   const [style, setStyle] = useState('detailed');
@@ -427,7 +429,9 @@ export default function AppearanceFooter() {
                   <SelectItem value="menu">Menu Links</SelectItem>
                   <SelectItem value="contact">Contact Info</SelectItem>
                   <SelectItem value="social">Social Links</SelectItem>
-                  <SelectItem value="newsletter">Newsletter Form</SelectItem>
+                  {isEnabled('newsletter') && (
+                    <SelectItem value="newsletter">Newsletter Form</SelectItem>
+                  )}
                   <SelectItem value="custom">Custom HTML</SelectItem>
                 </SelectContent>
               </Select>