Add AnnotationDto and AnnotationService for API integration

Added `AnnotationDto` to represent signature annotation positions,
including properties for ID, page, and coordinates, with detailed
documentation on coordinate systems. Implemented `AnnotationService`
to fetch annotation data from an API endpoint, using `HttpClient`
and supporting cancellation tokens. Registered `AnnotationService`
in the DI container to enable its use across the application.
Provided development setup details for working with fake data
via YARP-proxied routes.

EnvelopeGenerator.ReceiverUI/Models/AnnotationDto.cs

@@ -0,0 +1,29 @@
+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),
+/// 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>
+/// <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>
+/// </summary>
+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 hundredths of an inch 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>
+    public double? Y { get; init; }
+}