Add WarningView

ViewModels/WarningViewModel.cs

@@ -0,0 +1,25 @@
+#nullable enable
+
+using DialogHostAvalonia;
+
+namespace ReceiptPDFBuilder.ViewModels;
+
+class WarningViewModel
+{
+    private string _error;
+
+    public WarningViewModel(string error)
+    {
+        _error = error;
+    }
+
+    public string Error
+    {
+        get => _error;
+    }
+
+    public void Close()
+    {
+        DialogHost.Close("DialogHost", null);
+    }
+}