Merge branch 'feat/self-update'

Self-update for app and installer. Integrates cleanly with the
worker-log-footer feature that landed on main in parallel — the
shell VM now carries both worker-log state and update-check state,
and MainWindow hosts both the update banner and the footer log line.

Conflict resolved in IslandsShellViewModel.cs: kept nullable property
types from main's test-only parameterless constructor work, and added
the UpdateCheck property exposing the injected service.

src/ClaudeDo.Ui/Services/InstallerLocator.cs

@@ -0,0 +1,47 @@
+namespace ClaudeDo.Ui.Services;
+
+public sealed class InstallerLocator
+{
+    private const string InstallJson = "install.json";
+    private const string InstallerExe = "ClaudeDo.Installer.exe";
+    private const string UninstallerSubdir = "uninstaller";
+
+    public string? Find()
+        => FindByWalkingUp(AppContext.BaseDirectory) ?? FindByRegistry();
+
+    public string? FindByWalkingUp(string startDir)
+    {
+        var dir = new DirectoryInfo(startDir);
+        while (dir is not null)
+        {
+            var manifest = Path.Combine(dir.FullName, InstallJson);
+            if (File.Exists(manifest))
+            {
+                var candidate = Path.Combine(dir.FullName, UninstallerSubdir, InstallerExe);
+                return File.Exists(candidate) ? candidate : null;
+            }
+            dir = dir.Parent;
+        }
+        return null;
+    }
+
+    [System.Runtime.Versioning.SupportedOSPlatform("windows")]
+    public string? FindByRegistry()
+    {
+        if (!OperatingSystem.IsWindows()) return null;
+
+        try
+        {
+            using var key = Microsoft.Win32.Registry.LocalMachine
+                .OpenSubKey(@"Software\Microsoft\Windows\CurrentVersion\Uninstall\ClaudeDo");
+            var location = key?.GetValue("InstallLocation") as string;
+            if (string.IsNullOrEmpty(location)) return null;
+            var candidate = Path.Combine(location, UninstallerSubdir, InstallerExe);
+            return File.Exists(candidate) ? candidate : null;
+        }
+        catch
+        {
+            return null;
+        }
+    }
+}

src/ClaudeDo.Ui/Services/UpdateCheckService.cs

@@ -0,0 +1,73 @@
+using ClaudeDo.Releases;
+using CommunityToolkit.Mvvm.ComponentModel;
+
+namespace ClaudeDo.Ui.Services;
+
+public enum UpdateCheckStatus
+{
+    NeverChecked,
+    CheckFailed,
+    UpToDate,
+    UpdateAvailable,
+}
+
+public sealed partial class UpdateCheckService : ObservableObject
+{
+    private readonly IReleaseClient _releases;
+
+    [ObservableProperty] private bool _isUpdateAvailable;
+    [ObservableProperty] private string? _latestVersion;
+    [ObservableProperty] private string _currentVersion;
+    [ObservableProperty] private bool _isChecking;
+    [ObservableProperty] private UpdateCheckStatus _lastCheckStatus = UpdateCheckStatus.NeverChecked;
+
+    public UpdateCheckService(IReleaseClient releases, string currentVersion)
+    {
+        _releases = releases;
+        _currentVersion = currentVersion;
+    }
+
+    public async Task CheckNowAsync(CancellationToken ct)
+    {
+        IsChecking = true;
+        try
+        {
+            GiteaRelease? rel;
+            try
+            {
+                rel = await _releases.GetLatestReleaseAsync(ct);
+            }
+            catch
+            {
+                LastCheckStatus = UpdateCheckStatus.CheckFailed;
+                IsUpdateAvailable = false;
+                return;
+            }
+
+            if (rel is null)
+            {
+                LastCheckStatus = UpdateCheckStatus.CheckFailed;
+                IsUpdateAvailable = false;
+                return;
+            }
+
+            var latest = (rel.TagName ?? "").TrimStart('v', 'V');
+            var cmp = VersionComparer.Compare(latest, CurrentVersion);
+            if (cmp.IsNewer)
+            {
+                LatestVersion = latest;
+                IsUpdateAvailable = true;
+                LastCheckStatus = UpdateCheckStatus.UpdateAvailable;
+            }
+            else
+            {
+                IsUpdateAvailable = false;
+                LastCheckStatus = UpdateCheckStatus.UpToDate;
+            }
+        }
+        finally
+        {
+            IsChecking = false;
+        }
+    }
+}