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/Views/Controls/FeatureTabBase.xaml

@@ -0,0 +1,33 @@
+<UserControl x:Class="SharepointToolbox.Views.Controls.FeatureTabBase"
+             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:loc="clr-namespace:SharepointToolbox.Localization">
+  <Grid>
+    <Grid.RowDefinitions>
+      <RowDefinition Height="*" />
+      <RowDefinition Height="Auto" />
+    </Grid.RowDefinitions>
+
+    <!-- Placeholder content — Phase 2+ replaces Row 0 -->
+    <TextBlock Grid.Row="0"
+               Text="{Binding Source={x:Static loc:TranslationSource.Instance}, Path=[tab.comingsoon]}"
+               HorizontalAlignment="Center" VerticalAlignment="Center" />
+
+    <!-- Per-tab progress/cancel strip (shown only when IsRunning) -->
+    <Grid Grid.Row="1" Margin="8,4"
+          Visibility="{Binding IsRunning, Converter={StaticResource BoolToVisibilityConverter}}">
+      <Grid.ColumnDefinitions>
+        <ColumnDefinition Width="*" />
+        <ColumnDefinition Width="Auto" />
+        <ColumnDefinition Width="Auto" />
+      </Grid.ColumnDefinitions>
+      <ProgressBar Grid.Column="0" Height="16" Minimum="0" Maximum="100"
+                   Value="{Binding ProgressValue}" />
+      <TextBlock Grid.Column="1" Margin="8,0" VerticalAlignment="Center"
+                 Text="{Binding StatusMessage}" />
+      <Button Grid.Column="2"
+              Content="{Binding Source={x:Static loc:TranslationSource.Instance}, Path=[btn.cancel]}"
+              Command="{Binding CancelCommand}" Width="70" />
+    </Grid>
+  </Grid>
+</UserControl>

SharepointToolbox/Views/Controls/FeatureTabBase.xaml.cs

@@ -0,0 +1,11 @@
+using System.Windows.Controls;
+
+namespace SharepointToolbox.Views.Controls;
+
+public partial class FeatureTabBase : UserControl
+{
+    public FeatureTabBase()
+    {
+        InitializeComponent();
+    }
+}