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/Constants/UnitOfLength.cs

@@ -0,0 +1,65 @@
+namespace EnvelopeGenerator.ReceiverUI.Models.Constants;
+
+/// <summary>
+/// Represents the unit of measurement for coordinate values in signature positioning.
+/// Used for converting coordinates between different systems (GdPicture14, PDF.js, iText7).
+/// </summary>
+public enum UnitOfLength
+{
+    /// <summary>
+    /// Inch unit (1 inch = 25.4 mm).
+    /// This is the native unit used by GdPicture14 (EnvelopeGenerator.Form - Legacy VB.NET app).
+    /// Database stores all coordinates (X, Y, Width, Height) in INCHES.
+    /// </summary>
+    /// <remarks>
+    /// <b>Source:</b> GdPicture14.Annotations.AnnotationStickyNote uses INCHES natively.
+    /// <br/>
+    /// <b>Evidence:</b> VB.NET code directly assigns database values to annotation properties without conversion:
+    /// <code>
+    /// oAnnotation.Left = CSng(pElement.X)   ' Direct assignment → INCHES
+    /// oAnnotation.Top = CSng(pElement.Y)
+    /// </code>
+    /// <b>Standard Page Dimensions:</b>
+    /// <list type="bullet">
+    ///   <item>A4: 8.27" × 11.69" (210mm × 297mm)</item>
+    ///   <item>Letter: 8.5" × 11"</item>
+    /// </list>
+    /// </remarks>
+    Inch = 0,
+
+    /// <summary>
+    /// PDF Point unit (1 point = 1/72 inch).
+    /// This is the standard unit used by PDF specification and PDF.js viewer.
+    /// </summary>
+    /// <remarks>
+    /// <b>Definition:</b> According to PDF specification and Microsoft documentation:
+    /// <br/>
+    /// <i>"PDF pages are sized in point units. 1 pt == 1/72 inch"</i>
+    /// <br/><br/>
+    /// <b>Conversion Formula:</b>
+    /// <code>
+    /// points = inches * 72.0
+    /// inches = points / 72.0
+    /// </code>
+    /// <b>Important:</b> Point ≠ Pixel!
+    /// <list type="bullet">
+    ///   <item><b>Point (pt):</b> Device-independent unit (always 1/72 inch)</item>
+    ///   <item><b>Pixel (px):</b> Device-dependent unit (varies with screen DPI)</item>
+    ///   <item>At 72 DPI: 1 point = 1 pixel (coincidence)</item>
+    ///   <item>At 96 DPI: 1 point ≈ 1.33 pixels</item>
+    ///   <item>At 300 DPI: 1 point ≈ 4.17 pixels</item>
+    /// </list>
+    /// <b>Standard Page Dimensions (in points):</b>
+    /// <list type="bullet">
+    ///   <item>A4: 595 × 842 points (8.27" × 11.69" × 72)</item>
+    ///   <item>Letter: 612 × 792 points (8.5" × 11" × 72)</item>
+    /// </list>
+    /// <b>Usage in EnvelopeGenerator:</b>
+    /// <list type="bullet">
+    ///   <item>PDF.js viewer expects coordinates in points</item>
+    ///   <item>iText7 library uses points for PDF manipulation</item>
+    ///   <item>PSPDFKit (Web) uses points for annotation placement</item>
+    /// </list>
+    /// </remarks>
+    Point
+}

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)}'.")
+            };
+        }
+    }
 }