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/EmailTemplate.cs

@@ -4,42 +4,32 @@ using EnvelopeGenerator.Domain.Interfaces;
 using System.ComponentModel;
 using System.ComponentModel.DataAnnotations;
 using System.ComponentModel.DataAnnotations.Schema;
-#if NETFRAMEWORK
-#endif
 
 namespace EnvelopeGenerator.Domain.Entities
-#if NET
-    ;
-#elif NETFRAMEWORK
 {
-#endif
+    [Table("TBSIG_EMAIL_TEMPLATE", Schema = "dbo")]
+    public class EmailTemplate : IEntity, IHasAddedWhen, IHasChangedWhen
+    {
+        [Key]
+        [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
+        [Column("GUID")]
+        public int Id { get; set; }
 
-[Table("TBSIG_EMAIL_TEMPLATE", Schema = "dbo")]
-public class EmailTemplate : IEntity, IHasAddedWhen, IHasChangedWhen
-{
-    [Key]
-    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
-    [Column("GUID")]
-    public int Id { get; set; }
+        [Column("NAME", TypeName = "nvarchar(64)")]
+        public string Name { get; set; }
 
-    [Column("NAME", TypeName = "nvarchar(64)")]
-    public string Name { get; set; }
+        [Column("BODY", TypeName = "nvarchar(max)")]
+        public string Body { get; set; }
 
-    [Column("BODY", TypeName = "nvarchar(max)")]
-    public string Body { get; set; }
+        [Column("SUBJECT", TypeName = "nvarchar(512)")]
+        public string Subject { get; set; }
 
-    [Column("SUBJECT", TypeName = "nvarchar(512)")]
-    public string Subject { get; set; }
+        [Required]
+        [Column("ADDED_WHEN", TypeName = "datetime")]
+        [DefaultValue("GETDATE()")]
+        public DateTime AddedWhen { get; set; }
 
-    [Required]
-    [Column("ADDED_WHEN", TypeName = "datetime")]
-    [DefaultValue("GETDATE()")]
-    public DateTime AddedWhen { get; set; }
-
-    [Column("CHANGED_WHEN", TypeName = "datetime")]
-    public DateTime? ChangedWhen { get; set; }
-}
-
-#if NETFRAMEWORK
+        [Column("CHANGED_WHEN", TypeName = "datetime")]
+        public DateTime? ChangedWhen { get; set; }
     }
-#endif
+}