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/App.tsx

@@ -238,6 +238,7 @@ import PushConfiguration from '@/routes/Settings/Notifications/PushConfiguration
 import EmailCustomization from '@/routes/Settings/Notifications/EmailCustomization';
 import EditTemplate from '@/routes/Settings/Notifications/EditTemplate';
 import SettingsDeveloper from '@/routes/Settings/Developer';
+import SettingsModules from '@/routes/Settings/Modules';
 import AppearanceIndex from '@/routes/Appearance';
 import AppearanceGeneral from '@/routes/Appearance/General';
 import AppearanceHeader from '@/routes/Appearance/Header';
@@ -551,6 +552,7 @@ function AppRoutes() {
       <Route path="/settings/notifications/edit-template" element={<EditTemplate />} />
       <Route path="/settings/brand" element={<SettingsIndex />} />
       <Route path="/settings/developer" element={<SettingsDeveloper />} />
+      <Route path="/settings/modules" element={<SettingsModules />} />
       
       {/* Appearance */}
       <Route path="/appearance" element={<AppearanceIndex />} />