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/ApiServiceBase.cs

@@ -37,7 +37,13 @@ public abstract class ApiServiceBase
                 var errorBody = await response.Content.ReadAsStringAsync(ct);
                 Logger.LogWarning("GET {Endpoint} failed: {Status} - {Body}",
                     endpoint, (int)response.StatusCode, errorBody);
-                return ApiResponse<T>.Failure((int)response.StatusCode, errorBody);
+
+                // Versuche den Body trotzdem zu deserialisieren —
+                // die API gibt bei 401/404 oft strukturierte JSON-Antworten zurück
+                // (z.B. ReceiverAuthResponse mit ErrorMessage + Status)
+                var errorData = await TryDeserializeAsync<T>(response, ct);
+
+                return ApiResponse<T>.Failure((int)response.StatusCode, errorBody, errorData);
             }
 
             var data = await response.Content.ReadFromJsonAsync<T>(cancellationToken: ct);
@@ -70,7 +76,10 @@ public abstract class ApiServiceBase
                 var errorBody = await response.Content.ReadAsStringAsync(ct);
                 Logger.LogWarning("POST {Endpoint} failed: {Status} - {Body}",
                     endpoint, (int)response.StatusCode, errorBody);
-                return ApiResponse<TResponse>.Failure((int)response.StatusCode, errorBody);
+
+                var errorData = await TryDeserializeAsync<TResponse>(response, ct);
+
+                return ApiResponse<TResponse>.Failure((int)response.StatusCode, errorBody, errorData);
             }
 
             var data = await response.Content.ReadFromJsonAsync<TResponse>(cancellationToken: ct);
@@ -107,4 +116,26 @@ public abstract class ApiServiceBase
             return ApiResponse.Failure(0, "Verbindung zum Server fehlgeschlagen.");
         }
     }
+
+    /// <summary>
+    /// Versucht den Response-Body als JSON zu deserialisieren.
+    /// Gibt null zurück wenn es nicht klappt (z.B. bei HTML-Fehlerseiten).
+    /// </summary>
+    private static async Task<T?> TryDeserializeAsync<T>(HttpResponseMessage response, CancellationToken ct)
+    {
+        try
+        {
+            // Nur versuchen wenn der Content-Type JSON ist
+            if (response.Content.Headers.ContentType?.MediaType == "application/json")
+            {
+                return await response.Content.ReadFromJsonAsync<T>(cancellationToken: ct);
+            }
+        }
+        catch
+        {
+            // Ignorieren — der Body war kein valides JSON
+        }
+
+        return default;
+    }
 }