Standardize coordinate system to use INCHES

Updated the digital document signing system to use INCHES as the standard unit for annotations and signatures, aligning with GdPicture14's native format. Previously used DevExpress units (1/100 inch) and other formats have been replaced.

- Updated `AnnotationDto` to reflect the new coordinate system.
- Introduced `SignatureDto` for signature positions, deprecating `AnnotationDto`.
- Added conversion formulas for transforming coordinates between INCHES and other systems (DevExpress, PDF Points, PDF.js, etc.).
- Added a unit comparison table and A4 page dimensions in various units.
- Introduced a new read-only PDF.js viewer for envelopes (`/envelope/{EnvelopeKey}`).

These changes improve consistency, simplify conversions, and align with modern tools like PSPDFKit and iText7.

EnvelopeGenerator.ReceiverUI/Models/AnnotationDto.cs

@@ -3,15 +3,17 @@ namespace EnvelopeGenerator.ReceiverUI.Models;
 /// <summary>
 /// Represents a pre-assigned signature annotation position on a specific page.
 /// <br/><br/>
-/// <b>Coordinate unit (X, Y):</b> Hundredths of an inch (1/100 inch ? 2.83 PDF points),
+/// <b>Coordinate unit (X, Y):</b> Inches (GdPicture14 native unit),
 /// origin at the <b>top-left</b> corner of the page, both axes increase downward/rightward.
-/// This matches the DevExpress XtraReports coordinate system (<see cref="System.Drawing.RectangleF"/>).
 /// <br/><br/>
-/// <b>Difference from PSPDFKit:</b> Same origin (top-left) and direction, but PSPDFKit uses PDF points (1/72 inch).
-/// Convert: <c>xDX = xPsPdf * (100.0 / 72.0)</c>
+/// <b>Conversion to DevExpress:</b> Multiply by 100 (DX uses 1/100 inch).
+/// Convert: <c>xDX = xInches * 100.0</c>
 /// <br/>
-/// <b>Difference from GDPicture:</b> GDPicture uses PDF points with <b>bottom-left</b> origin (PDF standard); Y is flipped.
-/// Convert: <c>yDX = (pageHeightPt - yGD - elemHeightPt) * (100.0 / 72.0)</c>
+/// <b>Conversion to PDF Points:</b> Multiply by 72 (1 inch = 72 points).
+/// Convert: <c>xPt = xInches * 72.0</c>
+/// <br/>
+/// <b>Y-axis for PDF (bottom-left origin):</b> Flip required for iText7.
+/// Convert: <c>yPt = (pageHeightInches - yInches - elemHeightInches) * 72.0</c>
 /// </summary>
 [Obsolete("Use SignatureDto with SignatureService.")]
 public record AnnotationDto
@@ -22,9 +24,9 @@ public record AnnotationDto
     /// <summary>1-based page number within the document.</summary>
     public int Page { get; init; }
 
-    /// <summary>Horizontal position in hundredths of an inch from the left edge of the page.</summary>
+    /// <summary>Horizontal position in INCHES from the left edge of the page.</summary>
     public double X { get; init; }
 
-    /// <summary>Vertical position in hundredths of an inch from the top edge of the page.</summary>
+    /// <summary>Vertical position in INCHES from the top edge of the page.</summary>
     public double Y { get; init; }
 }

EnvelopeGenerator.ReceiverUI/Models/SignatureDto.cs

@@ -1,13 +1,22 @@
 namespace EnvelopeGenerator.ReceiverUI.Models;
 
+/// <summary>
+/// Represents a signature position on a PDF page.
+/// Coordinates stored in INCHES (GdPicture14 native unit).
+/// Origin: Top-left corner, X increases right, Y increases down.
+/// </summary>
 public class SignatureDto
 {
+    /// <summary>Unique identifier.</summary>
     public int Id { get; set; }
 
+    /// <summary>Horizontal position in INCHES from left edge.</summary>
     public double X { get; set; }
 
+    /// <summary>Vertical position in INCHES from top edge.</summary>
     public double Y { get; set; }
 
+    /// <summary>1-based page number.</summary>
     public int Page { get; set; }
 
 }