feat(01-07): add SettingsView and wire into MainWindow Settings tab

- Create Views/Tabs/SettingsView.xaml (UserControl with language ComboBox en/fr, DataFolder TextBox and Browse button using TranslationSource)
- Create Views/Tabs/SettingsView.xaml.cs (DI constructor injection of SettingsViewModel, LoadAsync on Loaded)
- Update MainWindow.xaml to add xmlns:views namespace and clear placeholder TextBlock from SettingsTabItem
- Register SettingsView as Transient in DI; resolve and set as SettingsTabItem.Content from MainWindow constructor
- All 42 unit tests pass, 0 build errors

SharepointToolbox/Views/Tabs/SettingsView.xaml.cs

@@ -0,0 +1,14 @@
+using System.Windows.Controls;
+using SharepointToolbox.ViewModels.Tabs;
+
+namespace SharepointToolbox.Views.Tabs;
+
+public partial class SettingsView : UserControl
+{
+    public SettingsView(SettingsViewModel viewModel)
+    {
+        InitializeComponent();
+        DataContext = viewModel;
+        Loaded += async (_, _) => await viewModel.LoadAsync();
+    }
+}