Refactor email account handling for dynamic resolution

Reintroduced `EmailAccountDto` with conditional compilation to support both .NET and non-.NET environments. Updated `IEmailService` to accept `EmailAccountDto` as the sender, replacing reliance on pre-configured SMTP credentials.

Added `GetSenderQuery` and its handler to dynamically resolve email accounts based on `Id` or `Username`. Introduced `GetSenderQueryValidator` for validation, ensuring proper usage of the query.

Modified `SendEmailCommand` to include sender resolution via MediatR. Updated `OutgoingEmailEvent` to include sender information and adjusted `OutgoingEmailConsumer` and `LimilabsEmailService` to use the dynamically resolved sender.

Updated `EmailMappingProfile` to ignore the `Sender` property during mapping. Replaced `Name` with `Id` in `appsettings.Secrets.json` for email accounts. Removed the old `EmailAccountDto` folder and performed general cleanup and restructuring.

src/core/DigitalData.MessagingService.Application/Common/Mappings/EmailMappingProfile.cs

@@ -12,8 +12,10 @@ public class EmailMappingProfile : Profile
     public EmailMappingProfile()
     {
         // SendEmailCommand -> OutgoingEmailEvent
+        // Sender is resolved via MediatR in the handler and set separately after mapping.
         CreateMap<SendEmailCommand, OutgoingEmailEvent>()
             .ForMember(dest => dest.Id, opt => opt.MapFrom(_ => Guid.NewGuid()))
-            .ForMember(dest => dest.QueuedAt, opt => opt.MapFrom(_ => DateTime.Now));
+            .ForMember(dest => dest.QueuedAt, opt => opt.MapFrom(_ => DateTime.Now))
+            .ForMember(dest => dest.Sender, opt => opt.Ignore());
     }
 }