Add support for signed PDF previews

Integrated `PdfSignatureService` to enable fetching and previewing signed PDFs for completed envelopes. Updated `SenderPage.razor` to dynamically adjust popup headers and filenames based on the envelope's status. Enhanced `OpenPreviewAsync` to conditionally fetch signed or original PDFs.

Registered `PdfSignatureService` and DevExpress PDF Viewer in `Program.cs`. Implemented `PdfSignatureService` to handle API calls for retrieving signed PDFs with error handling.

EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Services/PdfSignatureService.cs

@@ -0,0 +1,24 @@
+using System.Net;
+using System.Net.Http.Json;
+
+namespace EnvelopeGenerator.Server.Client.Services;
+
+public class PdfSignatureService(IHttpClientFactory httpClientFactory)
+{
+    /// <summary>
+    /// Calls POST /api/PdfSignature with the envelope id and returns the fully signed PDF bytes.
+    /// </summary>
+    public async Task<byte[]> GetSignedPdfAsync(int envelopeId, CancellationToken cancel = default)
+    {
+        using var http = httpClientFactory.CreateClient("EnvelopeGenerator.Server");
+        var response = await http.PostAsJsonAsync("/api/PdfSignature", new { Id = envelopeId }, cancel);
+
+        if (!response.IsSuccessStatusCode)
+            throw new HttpRequestException(
+                $"Failed to load signed PDF. Status: {(int)response.StatusCode} ({response.ReasonPhrase ?? "Unknown"})",
+                null,
+                response.StatusCode);
+
+        return await response.Content.ReadAsByteArrayAsync(cancel);
+    }
+}