Refactor services to use IHttpClientFactory

Refactored `DocReceiverElementService` and `EnvelopeService` to use `IHttpClientFactory` instead of directly injecting `HttpClient`, improving flexibility and testability.

Updated constructors to accept `IHttpClientFactory` and replaced direct `HttpClient` usage with named clients (`"EnvelopeGenerator.Server"`). Adjusted methods to use the factory-created clients for HTTP requests.

Added `using Microsoft.Extensions.Options;` in `Program.cs` and registered `EnvelopeService` and `DocReceiverElementService` in the dependency injection container for proper resolution. Clarified their usage in SSR scenarios with comments.

Removed redundant `using` directives and aligned imports with the updated implementation.

These changes enhance maintainability, scalability, and testability by leveraging `IHttpClientFactory` for better HTTP client management and dependency injection.

EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Services/DocReceiverElementService.cs

@@ -4,13 +4,15 @@ using EnvelopeGenerator.Server.Client.Models;
 
 namespace EnvelopeGenerator.Server.Client.Services;
 
-public class DocReceiverElementService(HttpClient http)
+public class DocReceiverElementService(IHttpClientFactory clientFactory)
 {
     private static readonly JsonSerializerOptions _jsonOptions = new(JsonSerializerDefaults.Web);
 
     public async Task<IReadOnlyList<SignatureDto>> GetAsync(string envelopeKey, CancellationToken cancel = default)
     {
         var url = $"/api/DocReceiverElement/{Uri.EscapeDataString(envelopeKey)}";
+
+        var http = clientFactory.CreateClient("EnvelopeGenerator.Server");
         var response = await http.GetAsync(url, cancel);
 
         if (!response.IsSuccessStatusCode)