Add UnitOfLength enum and enhance SignatureDto immutability

Introduced the `UnitOfLength` enum to represent measurement units
(Inch and Point) for signature positioning, with detailed
documentation and conversion logic.

Updated `SignatureDto` to use `init` accessors for immutability,
added backing fields for `X` and `Y` with conversion support, and
introduced the `Factor` property to handle unit conversions.

Added a `Convert` method to enable switching between units of
length and improved extensibility for future `SenderAppType`
support. Enhanced code readability and maintainability with
detailed comments and remarks.

EnvelopeGenerator.ReceiverUI/Models/SignatureDto.cs

@@ -10,16 +10,59 @@ namespace EnvelopeGenerator.ReceiverUI.Models;
 public class SignatureDto
 {
     /// <summary>Unique identifier.</summary>
-    public int Id { get; set; }
+    public int Id { get; init; }
+
+    private double _x;
+    private double _y;
 
     /// <summary>Horizontal position in INCHES from left edge.</summary>
-    public double X { get; set; }
+    public double X 
+    { 
+        get => _x * Factor;
+        init => _x = value; 
+    }
 
     /// <summary>Vertical position in INCHES from top edge.</summary>
-    public double Y { get; set; }
+    public double Y 
+    { 
+        get => _y * Factor;
+        init => _y = value; 
+    }
 
     /// <summary>1-based page number.</summary>
-    public int Page { get; set; }
+    public int Page { get; init; }
 
-    public SenderAppType SenderAppType { get; set; }
+    /// <summary>Sender application type that created this signature.</summary>
+    public SenderAppType SenderAppType { get; init; }
+
+    private UnitOfLength _unitOfLength;
+
+    public SignatureDto Convert(UnitOfLength unitOfLength)
+    {
+        _unitOfLength = unitOfLength;
+        return this;
+    }
+
+    public double Factor
+    {
+        get
+        {
+            if (SenderAppType != SenderAppType.LegacyFormApp)
+            {
+                throw new NotImplementedException(
+                    $"SenderAppType '{SenderAppType}' is not yet implemented. " +
+                    $"Currently, only '{nameof(SenderAppType.LegacyFormApp)}' is supported. " +
+                    $"Future implementations will handle '{nameof(SenderAppType.ReceiverUIBlazorApp)}' and other types.");
+            }
+
+            // LegacyFormApp uses GdPicture14 with INCHES
+            return _unitOfLength switch
+            {
+                UnitOfLength.Inch => 1.0,   // No conversion needed: INCHES → INCHES
+                UnitOfLength.Point => 72.0, // INCHES → PDF Points: 1 inch = 72 points (PDF standard, NOT pixels!)
+                _ => throw new InvalidOperationException(
+                    $"Unknown UnitOfLength: {_unitOfLength}. Expected '{nameof(UnitOfLength.Inch)}' or '{nameof(UnitOfLength.Point)}'.")
+            };
+        }
+    }
 }