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/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; }
 
 }