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/InstallModeDetector.cs

@@ -0,0 +1,41 @@
+namespace ClaudeDo.Installer.Core;
+
+public sealed record DetectedState(
+    InstallerMode Mode,
+    InstallManifest? Existing,
+    GiteaRelease? LatestRelease,
+    string? LatestVersion);
+
+public sealed class InstallModeDetector
+{
+    private readonly IReleaseClient _releases;
+
+    public InstallModeDetector(IReleaseClient releases)
+    {
+        _releases = releases;
+    }
+
+    public async Task<DetectedState> DetectAsync(string installDir, CancellationToken ct)
+    {
+        var manifest = InstallManifestStore.TryRead(installDir);
+        if (manifest is null)
+            return new DetectedState(InstallerMode.FreshInstall, null, null, null);
+
+        var release = await _releases.GetLatestReleaseAsync(ct);
+        if (release is null)
+            return new DetectedState(InstallerMode.Config, manifest, null, null);
+
+        var latestVersion = release.TagName.TrimStart('v', 'V');
+        if (IsNewer(latestVersion, manifest.Version))
+            return new DetectedState(InstallerMode.Update, manifest, release, latestVersion);
+
+        return new DetectedState(InstallerMode.Config, manifest, release, latestVersion);
+    }
+
+    private static bool IsNewer(string latest, string current)
+    {
+        if (!Version.TryParse(latest, out var lv)) return false;
+        if (!Version.TryParse(current, out var cv)) return false;
+        return lv > cv;
+    }
+}

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)
 }