Refactor entities: remove preprocessor directives, unify style

Large-scale cleanup of entity and interface classes:
- Removed obsolete #if NET/NETFRAMEWORK preprocessor blocks
- Unified class/property formatting and indentation
- Standardized use of data annotations
- Improved nullable reference type handling
- Simplified interfaces (IHasEnvelope, IHasReceiver)
- Removed redundant and dead code
- No changes to business logic or data model

These changes modernize and standardize the codebase for easier maintenance and .NET compatibility.

EnvelopeGenerator.Domain/Interfaces/IHasEnvelope.cs

@@ -1,22 +1,11 @@
 namespace EnvelopeGenerator.Domain.Interfaces
-#if NET
-    ;
-#elif NETFRAMEWORK
-    {
-#endif
-
-public interface IHasEnvelope
 {
-#if NET
-    public 
-#endif
+    public interface IHasEnvelope
+    {
         Entities.Envelope
-#if NET
-        ?
+#if nullable
+            ?
 #endif
             Envelope { get; set; }
-}
-
-#if NETFRAMEWORK
-}
-#endif
+    }
+}

EnvelopeGenerator.Domain/Interfaces/IHasReceiver.cs

@@ -1,22 +1,11 @@
 namespace EnvelopeGenerator.Domain.Interfaces
-#if NET
-    ;
-#elif NETFRAMEWORK
+{
+    public interface IHasReceiver
     {
-#endif
-
-public interface IHasReceiver
-    {
-#if NET
-    public 
-#endif
         Entities.Receiver
-#if NET
-        ?
+#if nullable
+            ?
 #endif
             Receiver { get; set; }
-}
-
-#if NETFRAMEWORK
-}
-#endif
+    }
+}