feat(06-03): add global site picker button and count label to toolbar

- Add Separator + Select Sites button (bound to OpenGlobalSitePickerCommand) to ToolBar
- Add TextBlock bound to GlobalSitesSelectedLabel for site count display
- Wire viewModel.OpenGlobalSitePickerDialog factory in MainWindow.xaml.cs using DI
- Add using SharepointToolbox.Core.Models for TenantProfile in code-behind

SharepointToolbox/MainWindow.xaml

@@ -22,6 +22,13 @@
       <Separator />
       <Button Content="{Binding Source={x:Static loc:TranslationSource.Instance}, Path=[toolbar.clear]}"
               Command="{Binding ClearSessionCommand}" />
+      <Separator />
+      <Button Content="{Binding Source={x:Static loc:TranslationSource.Instance}, Path=[toolbar.selectSites]}"
+              Command="{Binding OpenGlobalSitePickerCommand}"
+              ToolTip="{Binding Source={x:Static loc:TranslationSource.Instance}, Path=[toolbar.selectSites.tooltip]}" />
+      <TextBlock Text="{Binding GlobalSitesSelectedLabel}"
+                 VerticalAlignment="Center" Margin="6,0,0,0"
+                 Foreground="Gray" />
     </ToolBar>
 
     <!-- StatusBar: tenant name | operation status text | progress % -->