feat: implement module purchase dialog with subscription type selection, auto-renew, and dependency checks, replacing the dedicated purchase page.

src/backend/Zentral.API/Controllers/ModulesController.cs

@@ -244,7 +244,7 @@ public class ModulesController : ControllerBase
             Dependencies = module.GetDependencies().ToList(),
             RoutePath = module.RoutePath,
             IsAvailable = module.IsAvailable,
-            IsEnabled = module.IsCore || (module.Subscription?.IsValid() ?? false),
+            IsEnabled = module.IsCore || ((module.Subscription?.IsEnabled ?? false) && (module.Subscription?.IsValid() ?? false)),
             Subscription = module.Subscription != null ? MapSubscriptionToDto(module.Subscription) : null
         };
     }