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.

Controls/BottomBar.xaml.cs

@@ -0,0 +1,22 @@
+using System.Windows;
+using System.Windows.Controls;
+
+namespace ytLive.Controls;
+
+public partial class BottomBar : UserControl
+{
+    public BottomBar()
+    {
+        InitializeComponent();
+    }
+
+    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;
+        }
+    }
+}