Refactor envelope receiver auth flow and state handling

- Introduce IReceiverAuthService and ReceiverAuthService for all envelope receiver authentication and status API calls
- Add ReceiverAuthModel as a client-side DTO for API responses
- Refactor EnvelopeState to store all relevant fields and update via ApplyApiResponse
- Overhaul EnvelopePage.razor to use new service and state, with improved status handling and UI
- Enhance ApiResponse and ApiServiceBase to support structured error deserialization
- Register IReceiverAuthService in DI container

EnvelopeGenerator.ReceiverUI/EnvelopeGenerator.ReceiverUI.Client/Services/IReceiverAuthSerive.cs

@@ -0,0 +1,26 @@
+using EnvelopeGenerator.ReceiverUI.Client.Models;
+using EnvelopeGenerator.ReceiverUI.Client.Services.Base;
+
+namespace EnvelopeGenerator.ReceiverUI.Client.Services;
+
+/// <summary>
+/// Kommuniziert mit dem ReceiverAuthController der API.
+/// 
+/// Drei Methoden — eine pro Endpunkt:
+/// 1. GetStatusAsync    → GET  /api/receiverauth/{key}/status
+/// 2. SubmitAccessCodeAsync → POST /api/receiverauth/{key}/access-code
+/// 3. SubmitTfaCodeAsync    → POST /api/receiverauth/{key}/tfa
+/// </summary>
+public interface IReceiverAuthService
+{
+    /// <summary>Prüft den aktuellen Status des Empfänger-Flows</summary>
+    Task<ApiResponse<ReceiverAuthModel>> GetStatusAsync(string key, CancellationToken ct = default);
+
+    /// <summary>Sendet den Zugangscode zur Prüfung</summary>
+    Task<ApiResponse<ReceiverAuthModel>> SubmitAccessCodeAsync(
+        string key, string accessCode, bool preferSms, CancellationToken ct = default);
+
+    /// <summary>Sendet den TFA-Code (SMS oder Authenticator) zur Prüfung</summary>
+    Task<ApiResponse<ReceiverAuthModel>> SubmitTfaCodeAsync(
+        string key, string code, string type, CancellationToken ct = default);
+}