feat(installer): add Diagnose section to SettingsWindow

Re-runs the environment checks against the installed configuration
(worker.config.json + detected install dir) without blocking navigation
and without auto-running on window open, only on a "Recheck" click.

Extracted the check-row rendering and check-run logic (busy state,
summary, Recheck command) out of SystemCheckPage into a shared
Checks/CheckListViewModel + Checks/CheckListView, composed by both
SystemCheckPage (wizard) and the new DiagnosePage (settings) instead
of duplicating it.

src/ClaudeDo.Installer/Pages/DiagnosePage/DiagnosePageViewModel.cs

@@ -0,0 +1,50 @@
+using System.Windows.Controls;
+using ClaudeDo.Installer.Checks;
+using ClaudeDo.Installer.Core;
+using ClaudeDo.Installer.Localization;
+using CommunityToolkit.Mvvm.ComponentModel;
+
+namespace ClaudeDo.Installer.Pages.DiagnosePage;
+
+/// <summary>
+/// Re-runs the same environment checks as the wizard's SystemCheckPage, but against the
+/// installed configuration (worker.config.json + the detected install directory) instead of
+/// wizard defaults, and nothing here blocks navigation or auto-runs on load.
+/// </summary>
+public partial class DiagnosePageViewModel : ObservableObject, IInstallerPage
+{
+    private readonly InstallContext _sharedContext;
+    private readonly Func<InstallerWorkerConfig> _loadWorkerConfig;
+    private readonly InstallContext _installedContext = new();
+    private DiagnosePageView? _view;
+
+    public string Title => TrExtension.Localizer?["installer.diagnose.title"] ?? "Diagnose";
+    public string Icon => "";
+    public int Order => 5;
+    public bool ShowInWizard => false;
+    public bool ShowInSettings => true;
+    public UserControl View => _view ??= new DiagnosePageView { DataContext = this };
+
+    public CheckListViewModel Checks { get; }
+
+    public DiagnosePageViewModel(InstallContext sharedContext, Func<EnvironmentCheckService> checkServiceFactory, Func<InstallerWorkerConfig> loadWorkerConfig)
+    {
+        _sharedContext = sharedContext;
+        _loadWorkerConfig = loadWorkerConfig;
+        Checks = new CheckListViewModel(_installedContext, checkServiceFactory);
+    }
+
+    public Task LoadAsync()
+    {
+        var cfg = _loadWorkerConfig();
+        _installedContext.InstallDirectory = _sharedContext.InstallDirectory;
+        _installedContext.ClaudeBin = cfg.ClaudeBin;
+        _installedContext.SignalRPort = cfg.SignalRPort;
+        _installedContext.ExternalMcpPort = _sharedContext.ExternalMcpPort;
+        return Task.CompletedTask;
+    }
+
+    public Task ApplyAsync() => Task.CompletedTask;
+
+    public bool Validate() => true;
+}