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/Entities/Config.cs

@@ -2,30 +2,22 @@
 using System.ComponentModel.DataAnnotations.Schema;
 
 namespace EnvelopeGenerator.Domain.Entities
-#if NET
-    ;
-#elif NETFRAMEWORK
-    {
-#endif
-
-[Table("TBSIG_CONFIG", Schema = "dbo")]
-public class Config
 {
-    [Column("SENDING_PROFILE", TypeName = "int")]
-    [Required]
-    public int SendingProfile { get; set; }
+    [Table("TBSIG_CONFIG", Schema = "dbo")]
+    public class Config
+    {
+        [Column("SENDING_PROFILE", TypeName = "int")]
+        [Required]
+        public int SendingProfile { get; set; }
 
-    [Column("SIGNATURE_HOST", TypeName = "nvarchar(128)")]
-    [Required]
-    public string SignatureHost { get; set; }
+        [Column("SIGNATURE_HOST", TypeName = "nvarchar(128)")]
+        [Required]
+        public string SignatureHost { get; set; }
 
-    [Column("EXTERNAL_PROGRAM_NAME", TypeName = "nvarchar(30)")]
-    public string ExternalProgramName { get; set; }
+        [Column("EXTERNAL_PROGRAM_NAME", TypeName = "nvarchar(30)")]
+        public string ExternalProgramName { get; set; }
 
-    [Column("EXPORT_PATH", TypeName = "nvarchar(256)")]
-    public string ExportPath { get; set; }
-}
-
-#if NETFRAMEWORK
+        [Column("EXPORT_PATH", TypeName = "nvarchar(256)")]
+        public string ExportPath { get; set; }
     }
-#endif
+}