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/AnnotationDto.cs

@@ -0,0 +1,32 @@
+namespace EnvelopeGenerator.WebUI.Client.Models;
+
+/// <summary>
+/// Represents a pre-assigned signature annotation position on a specific page.
+/// <br/><br/>
+/// <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.
+/// <br/><br/>
+/// <b>Conversion to DevExpress:</b> Multiply by 100 (DX uses 1/100 inch).
+/// Convert: <c>xDX = xInches * 100.0</c>
+/// <br/>
+/// <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
+{
+    /// <summary>Unique identifier of the annotation.</summary>
+    public long Id { get; init; }
+
+    /// <summary>1-based page number within the document.</summary>
+    public int Page { get; init; }
+
+    /// <summary>Horizontal position in INCHES from the left edge of the page.</summary>
+    public double X { get; init; }
+
+    /// <summary>Vertical position in INCHES from the top edge of the page.</summary>
+    public double Y { get; init; }
+}