applicationId uz neni povinne

Credentials/IdokladCredentials.cs

@@ -10,6 +10,13 @@ public sealed record IdokladCredentials
 {
     public required string ClientId { get; init; }
     public required string ClientSecret { get; init; }
-    public required string ApplicationId { get; init; }
+
+    /// <summary>
+    /// Optional iDoklad ApplicationId (GUID from the developer portal). Only needed for partner
+    /// applications whose client_credentials grant requires application_id. When empty, the
+    /// service authenticates with client_id + client_secret only.
+    /// </summary>
+    public string? ApplicationId { get; init; }
+
     public required Language Language { get; init; }
 }