feat(installer): replace sync ModeDetector with async InstallModeDetector

Placeholder edit to App.xaml.cs to keep the project building until Task 11
wires the new async detector.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ClaudeDo.Installer/Core/InstallerMode.cs

@@ -1,19 +1,8 @@
-using System.IO;
-using ClaudeDo.Data;
-
 namespace ClaudeDo.Installer.Core;
 
-public enum InstallerMode { Wizard, Settings }
-
-public static class ModeDetector
+public enum InstallerMode
 {
-    public static InstallerMode Detect()
-    {
-        var root = Paths.AppDataRoot();
-        var workerConfig = Path.Combine(root, "worker.config.json");
-        var uiConfig = Path.Combine(root, "ui.config.json");
-        return File.Exists(workerConfig) && File.Exists(uiConfig)
-            ? InstallerMode.Settings
-            : InstallerMode.Wizard;
-    }
+    FreshInstall,   // No install.json present -> run full wizard
+    Update,         // install.json present, newer release available
+    Config,         // install.json present, no update (or API unreachable)
 }