feat: implement communications module with SMTP settings, email logging, and frontend UI

src/backend/Zentral.Domain/Interfaces/IEmailSender.cs

@@ -0,0 +1,7 @@
+namespace Zentral.Domain.Interfaces;
+
+public interface IEmailSender
+{
+    Task SendEmailAsync(string to, string subject, string body, bool isHtml = true);
+    Task SendEmailAsync(string to, string subject, string body, List<string> attachments, bool isHtml = true);
+}