claude refactorings, flutter best practices, platform dependent changes, general cleanup

lib/api/webuntis/webuntis_error.dart

@@ -0,0 +1,9 @@
+class WebuntisError implements Exception {
+  String message;
+  int code;
+
+  WebuntisError(this.message, this.code);
+
+  @override
+  String toString() => 'WebUntis ($code): $message';
+}