Make AnnotationDto properties non-nullable

Changed the `Page` property from `int?` to `int` to ensure it always has a value. Similarly, changed the `X` and `Y` properties from `double?` to `double` to enforce non-nullable values for horizontal and vertical positions.

EnvelopeGenerator.ReceiverUI/Models/AnnotationDto.cs

@@ -19,11 +19,11 @@ public record AnnotationDto
     public long Id { get; init; }
 
     /// <summary>1-based page number within the document.</summary>
-    public int? Page { get; init; }
+    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; }
+    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; }
+    public double Y { get; init; }
 }