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/Base/ApiResponse.cs

@@ -19,6 +19,14 @@ public record ApiResponse<T>
 
     public static ApiResponse<T> Failure(int statusCode, string? error = null)
         => new() { IsSuccess = false, StatusCode = statusCode, ErrorMessage = error };
+
+    /// <summary>
+    /// Failure mit deserialisiertem Body — für Fälle wo die API
+    /// bei 401/404 trotzdem ein strukturiertes JSON zurückgibt
+    /// (z.B. ReceiverAuthResponse mit ErrorMessage + Status).
+    /// </summary>
+    public static ApiResponse<T> Failure(int statusCode, string? error, T? data)
+        => new() { IsSuccess = false, StatusCode = statusCode, ErrorMessage = error, Data = data };
 }
 
 /// <summary>