Add envelope creation functionality

Introduced the ability to create envelopes with documents and receivers via a new `CreateAsync` method in `EnvelopeReceiverService`. Integrated this functionality into `EnvelopeSenderEditorPage.razor` with UI updates, including a save button spinner, validation checks, and a result popup for success or error feedback.

- Added `CreateAsync` method to handle `POST /api/EnvelopeReceiver` API calls.
- Injected `EnvelopeReceiverService` into `EnvelopeSenderEditorPage.razor`.
- Implemented save logic with validation for PDF upload, receivers, and signature fields.
- Added success and error popups for user feedback.
- Improved logging for envelope creation and validation warnings.
- Refactored save logic for better readability and maintainability.

EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Services/EnvelopeReceiverService.cs

@@ -2,6 +2,7 @@ using System.Net;
 using System.Net.Http;
 using System.Net.Http.Json;
 using System.Text.Json;
+using EnvelopeGenerator.Application.EnvelopeReceivers.Commands;
 using EnvelopeGenerator.Server.Client.Models;
 
 namespace EnvelopeGenerator.Server.Client.Services;
@@ -9,6 +10,7 @@ namespace EnvelopeGenerator.Server.Client.Services;
 /// <summary>
 /// Retrieves the <see cref="EnvelopeReceiverDto"/> for the authenticated receiver
 /// from <c>GET /api/EnvelopeReceiver/{envelopeKey}</c>.
+/// Also creates new envelopes via <c>POST /api/EnvelopeReceiver</c>.
 /// </summary>
 public class EnvelopeReceiverService(IHttpClientFactory httpClientFactory)
 {
@@ -37,4 +39,28 @@ public class EnvelopeReceiverService(IHttpClientFactory httpClientFactory)
 
         return await response.Content.ReadFromJsonAsync<EnvelopeReceiverDto>(_jsonOptions, cancel);
     }
+
+    /// <summary>
+    /// Creates a new envelope with document and receivers via <c>POST /api/EnvelopeReceiver</c>.
+    /// Requires sender authentication cookie to be present in the request.
+    /// </summary>
+    /// <exception cref="HttpRequestException">Thrown when the API request fails.</exception>
+    public async Task<CreateEnvelopeReceiverResponse?> CreateAsync(
+        CreateEnvelopeReceiverCommand request,
+        CancellationToken cancel = default)
+    {
+        using var http = httpClientFactory.CreateClient("EnvelopeGenerator.Server");
+        var response   = await http.PostAsJsonAsync("/api/EnvelopeReceiver", request, _jsonOptions, cancel);
+
+        if (!response.IsSuccessStatusCode)
+        {
+            var body = await response.Content.ReadAsStringAsync(cancel);
+            throw new HttpRequestException(
+                $"Fehler beim Erstellen des Umschlags. Status: {(int)response.StatusCode} – {body}",
+                null,
+                response.StatusCode);
+        }
+
+        return await response.Content.ReadFromJsonAsync<CreateEnvelopeReceiverResponse>(_jsonOptions, cancel);
+    }
 }