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/Services/CustomReportProvider.cs

@@ -0,0 +1,21 @@
+using DevExpress.XtraReports.UI;
+using DevExpress.XtraReports.Services;
+using EnvelopeGenerator.WebUI.Client.PredefinedReports;
+
+namespace EnvelopeGenerator.WebUI.Client.Services
+{
+    public class CustomReportProvider : IReportProviderAsync {
+        private readonly InMemoryReportStorageWebExtension reportStorage;
+
+        public CustomReportProvider(InMemoryReportStorageWebExtension reportStorage) {
+            this.reportStorage = reportStorage;
+        }
+
+        public Task<XtraReport> GetReportAsync(string id, ReportProviderContext context) {
+            if(reportStorage.TryGetReport(id, out var savedReport))
+                return Task.FromResult(savedReport);
+
+            return Task.FromResult(ReportsFactory.GetReport(id));
+        }
+    }
+}