refactor: extract BottomBar out of MainWindow.xaml — Phase 2, Commit 12

New Controls/ folder + namespace ytLive.Controls. BottomBar takes the
Grid.Row=3 footer: gear menu button (with its GearButton_Click handler moved to
the control's code-behind), centered stream-stats (bitrate/FPS/dropped/
duration/health), and the quality dropdown. All bindings inherit the window
DataContext; app-level styles resolve from Themes/Controls.xaml as before.

Zero behavior change; build 0 warnings; 246 pass, only the 2 known failures.
Docs: ViewModels/index.md tracker gains a Phase 2 section in the same commit.

MainWindow.xaml.cs

@@ -291,16 +291,6 @@ public partial class MainWindow : Window
         e.Handled = true; // the button's right-click context menu must not fire
     }
 
-    private void GearButton_Click(object sender, RoutedEventArgs e)
-    {
-        if (sender is Button { ContextMenu: { } menu } button)
-        {
-            menu.PlacementTarget = button;
-            menu.Placement = System.Windows.Controls.Primitives.PlacementMode.Bottom;
-            menu.IsOpen = true;
-        }
-    }
-
     private void BackToAbout_Click(object sender, RoutedEventArgs e)
     {
         if (DataContext is ViewModels.MainViewModel vm)