Add signature workflow models, services, and configurations

Introduced new models (`SignatureDto`, `SignatureCaptureDto`, `EnvelopeReceiverDto`) to support a signature-based workflow. Added services for handling API interactions (`SignatureService`, `AuthService`, `DocumentService`, `EnvelopeReceiverService`, `SignatureCacheService`).

Enhanced configuration with `ApiOptions` and `PdfViewerOptions`. Integrated DevExpress features with custom data connection providers, in-memory report storage, and font loading utilities.

Marked `AnnotationDto` and `AnnotationService` as `[Obsolete]` in favor of newer implementations. Added detailed documentation for coordinate systems, unit conversions, and usage scenarios.

EnvelopeGenerator.WebUI/EnvelopeGenerator.WebUI.Client/Models/Constants/SenderAppType.cs

@@ -0,0 +1,8 @@
+namespace EnvelopeGenerator.WebUI.Client.Models.Constants
+{
+    public enum SenderAppType
+    {
+        LegacyFormApp = 0,
+        ReceiverUIBlazorApp = 1
+    }
+}

EnvelopeGenerator.WebUI/EnvelopeGenerator.WebUI.Client/Models/Constants/UnitOfLength.cs

@@ -0,0 +1,65 @@
+namespace EnvelopeGenerator.WebUI.Client.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
+}