refactor(di): unify Application and Infrastructure DI registrations under a central method

- Added central AddEnvelopeGenerator extension to aggregate existing DI setups
- Introduced EGConfiguration for modular service registration
- Standardized configuration pattern for Application and Infrastructure layers
- Simplified distributed cache and localization registration

EnvelopeGenerator.Application/DependencyInjection.cs

@@ -61,6 +61,9 @@ public static class DependencyInjection
         // Add memory cache
         services.AddMemoryCache();
 
+        // Register mail services
+        services.AddScoped<IEnvelopeMailService, EnvelopeMailService>();
+
         return services;
     }
 }