feat(installer): self-update pre-flight before wizard

src/ClaudeDo.Installer/Views/SelfUpdatePromptWindow.xaml.cs

@@ -0,0 +1,42 @@
+using System.Windows;
+
+namespace ClaudeDo.Installer.Views;
+
+public enum SelfUpdateChoice { Update, Continue, Cancel }
+
+public partial class SelfUpdatePromptWindow : Window
+{
+    public SelfUpdateChoice Choice { get; private set; } = SelfUpdateChoice.Cancel;
+
+    public SelfUpdatePromptWindow(string currentVersion, string latestVersion)
+    {
+        InitializeComponent();
+        DetailText.Text = $"Installer v{latestVersion} is available (you are running v{currentVersion}). Update before continuing?";
+    }
+
+    public void ShowProgress(string text)
+    {
+        ProgressText.Text = text;
+        ProgressText.Visibility = Visibility.Visible;
+        UpdateBtn.IsEnabled = false;
+        ContinueBtn.IsEnabled = false;
+    }
+
+    private void UpdateBtn_Click(object sender, RoutedEventArgs e)
+    {
+        Choice = SelfUpdateChoice.Update;
+        DialogResult = true;
+    }
+
+    private void ContinueBtn_Click(object sender, RoutedEventArgs e)
+    {
+        Choice = SelfUpdateChoice.Continue;
+        DialogResult = true;
+    }
+
+    private void CancelBtn_Click(object sender, RoutedEventArgs e)
+    {
+        Choice = SelfUpdateChoice.Cancel;
+        DialogResult = false;
+    }
+}