feat(04-07): add Phase 4 localization, shared dialogs, and example CSV resources

- Add 80+ Phase 4 EN/FR localization keys to Strings.resx and Strings.fr.resx (tabs, transfer, bulkmembers, bulksites, folderstruct, templates, bulk-shared, folderbrowser)
- Add ResourceManager property accessors for all new keys to Strings.Designer.cs
- Create ConfirmBulkOperationDialog (XAML + code-behind) with Proceed/Cancel buttons
- Create FolderBrowserDialog (XAML + code-behind) with lazy-loading TreeView of SharePoint libraries/folders
- Bundle bulk_add_members.csv, bulk_create_sites.csv, folder_structure.csv as EmbeddedResource in csproj

SharepointToolbox/Views/Dialogs/ConfirmBulkOperationDialog.xaml.cs

@@ -0,0 +1,28 @@
+using System.Windows;
+
+namespace SharepointToolbox.Views.Dialogs;
+
+public partial class ConfirmBulkOperationDialog : Window
+{
+    public bool IsConfirmed { get; private set; }
+
+    public ConfirmBulkOperationDialog(string message)
+    {
+        InitializeComponent();
+        MessageText.Text = message;
+    }
+
+    private void Proceed_Click(object sender, RoutedEventArgs e)
+    {
+        IsConfirmed = true;
+        DialogResult = true;
+        Close();
+    }
+
+    private void Cancel_Click(object sender, RoutedEventArgs e)
+    {
+        IsConfirmed = false;
+        DialogResult = false;
+        Close();
+    }
+}