refactors and more UI

UI/MainWindow.axaml.cs

@@ -1,14 +1,21 @@
+using System;
+using System.Threading.Tasks;
 using Avalonia.Controls;
 using Avalonia.Styling;
+using Avalonia.Threading;
+using MercuryConverter.UI.Dialogs;
+using MercuryConverter.UI.Views;
 
 namespace MercuryConverter.UI;
 
-using MercuryConverter.UI.Views;
-
 public partial class MainWindow : Window
 {
+    public static MainWindow? Instance { get; private set; }
+    private bool initialShown = false;
+
     public MainWindow()
     {
+        Instance = this;
         InitializeComponent();
 
         // Force dark mode in designer
@@ -19,5 +26,17 @@ public partial class MainWindow : Window
 
         // Setup tab views
         SelectionControl.Content = new Selection();
+
+        // Show dialog on startup
+        Activated += OnActivated;
+    }
+
+    private void OnActivated(object? sender, EventArgs e)
+    {
+        if (initialShown) return;
+        initialShown = true;
+
+        Dialog.DialogContent = new Welcome().Content;
+        Dialog.IsOpen = true;
     }
 }