refactors and more UI

UI/Dialogs/Welcome.axaml.cs

@@ -0,0 +1,19 @@
+using System;
+using Avalonia.Controls;
+using Avalonia.Interactivity;
+using DialogHostAvalonia;
+
+namespace MercuryConverter.UI.Dialogs;
+
+public partial class Welcome : Window
+{
+    public Welcome()
+    {
+        InitializeComponent();
+    }
+
+    private void ClickHandler(object sender, RoutedEventArgs args)
+    {
+        MainWindow.Instance!.Dialog.DialogContent = new DataOpen().Content;
+    }
+}