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

src/backend/Zentral.Domain/Entities/ModuleSubscription.cs

@@ -72,9 +72,6 @@ public class ModuleSubscription : BaseEntity
     /// </summary>
     public bool IsValid()
     {
-        if (!IsEnabled)
-            return false;
-
         // Se non c'è data di scadenza, è valido (licenza perpetua o core module)
         if (!EndDate.HasValue)
             return true;