fix: Navigation issues - newsletter menu, coupon routing, module toggle

Navigation Fixes:
1. Newsletter submenu now hidden when module disabled
   - NavigationRegistry checks ModuleRegistry::is_enabled('newsletter')
   - Menu updates dynamically based on module status

2. Module toggle now updates navigation in real-time
   - Fixed toggle_module API to return success response (was returning error)
   - Navigation cache flushes and rebuilds when module toggled
   - Newsletter menu appears/disappears immediately after toggle

3. Coupon routes now activate Marketing menu (not Dashboard)
   - Added special case in useActiveSection for /coupons paths
   - Marketing menu stays active when viewing coupons
   - Submenu shows correct Marketing items (Newsletter, Coupons)

4. Dashboard menu no longer always shows active
   - Fixed by proper path matching in useActiveSection
   - Only active when on dashboard routes

Files Modified (4):
- includes/Compat/NavigationRegistry.php (already had newsletter check, added rebuild on flush)
- includes/Api/ModulesController.php (fixed toggle_module response)
- admin-spa/src/hooks/useActiveSection.ts (added /coupons special case)
- admin-spa/dist/app.js (rebuilt)

All 4 navigation issues resolved!

includes/Compat/NavigationRegistry.php

@@ -260,10 +260,12 @@ class NavigationRegistry {
     }
     
     /**
-     * Flush the navigation cache
+     * Flush navigation cache
      */
     public static function flush() {
         delete_option(self::NAV_OPTION);
+        // Rebuild immediately after flush
+        self::build_nav_tree();
     }
     
     /**