feat(01-06): build WPF shell — MainWindow XAML, ViewModels, LogPanelSink wiring

- Add FeatureTabBase UserControl with ProgressBar/TextBlock/CancelButton strip
  (Visibility bound to IsRunning, shown only during operations)
- Add MainWindowViewModel with TenantProfiles ObservableCollection, ConnectCommand,
  ClearSessionCommand, ManageProfilesCommand, ProgressUpdatedMessage subscription
- Add ProfileManagementViewModel wrapping ProfileService CRUD with input validation
- Add SettingsViewModel (extends FeatureViewModelBase) with language/folder settings
- Update MainWindow.xaml: DockPanel shell with Toolbar, TabControl (8 tabs), 150px
  RichTextBox LogPanel, StatusBar (tenant name | ProgressStatus | ProgressPercentage)
- MainWindow.xaml.cs: DI constructor, DataContext=viewModel, LoadProfilesAsync on Loaded
- App.xaml.cs: register all services, wire LogPanelSink after MainWindow resolved,
  register DispatcherUnhandledException and UnobservedTaskException global handlers
- App.xaml: add BoolToVisibilityConverter resource

SharepointToolbox/ViewModels/Tabs/SettingsViewModel.cs

@@ -0,0 +1,94 @@
+using System.Globalization;
+using Microsoft.Win32;
+using CommunityToolkit.Mvvm.Input;
+using Microsoft.Extensions.Logging;
+using SharepointToolbox.Core.Models;
+using SharepointToolbox.Localization;
+using SharepointToolbox.Services;
+
+namespace SharepointToolbox.ViewModels.Tabs;
+
+public partial class SettingsViewModel : FeatureViewModelBase
+{
+    private readonly SettingsService _settingsService;
+
+    private string _selectedLanguage = "en";
+    public string SelectedLanguage
+    {
+        get => _selectedLanguage;
+        set
+        {
+            if (_selectedLanguage == value) return;
+            _selectedLanguage = value;
+            OnPropertyChanged();
+            _ = ApplyLanguageAsync(value);
+        }
+    }
+
+    private string _dataFolder = string.Empty;
+    public string DataFolder
+    {
+        get => _dataFolder;
+        set
+        {
+            if (_dataFolder == value) return;
+            _dataFolder = value;
+            OnPropertyChanged();
+            _ = _settingsService.SetDataFolderAsync(value);
+        }
+    }
+
+    public RelayCommand BrowseFolderCommand { get; }
+
+    public SettingsViewModel(SettingsService settingsService, ILogger<FeatureViewModelBase> logger)
+        : base(logger)
+    {
+        _settingsService = settingsService;
+        BrowseFolderCommand = new RelayCommand(BrowseFolder);
+    }
+
+    public async Task LoadAsync()
+    {
+        var settings = await _settingsService.GetSettingsAsync();
+        _selectedLanguage = settings.Lang;
+        _dataFolder = settings.DataFolder;
+        OnPropertyChanged(nameof(SelectedLanguage));
+        OnPropertyChanged(nameof(DataFolder));
+    }
+
+    private async Task ApplyLanguageAsync(string code)
+    {
+        try
+        {
+            TranslationSource.Instance.CurrentCulture = new CultureInfo(code);
+            await _settingsService.SetLanguageAsync(code);
+        }
+        catch (Exception ex)
+        {
+            StatusMessage = ex.Message;
+        }
+    }
+
+    private void BrowseFolder()
+    {
+        // OpenFolderDialog is available in .NET 8+ via Microsoft.Win32
+        var dialog = new OpenFolderDialog
+        {
+            Title = "Select data output folder",
+            InitialDirectory = string.IsNullOrEmpty(_dataFolder)
+                ? Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments)
+                : _dataFolder
+        };
+
+        if (dialog.ShowDialog() == true)
+        {
+            DataFolder = dialog.FolderName;
+        }
+    }
+
+    protected override Task RunOperationAsync(CancellationToken ct, IProgress<OperationProgress> progress)
+    {
+        // Settings tab has no long-running operation
+        throw new NotSupportedException("Settings tab does not have a run operation.");
+    }
+}