refactor: extract SceneThumbnailStrip out of MainWindow.xaml — Phase 2, Commit 16

SceneThumbnailStrip.xaml(.cs) takes the Row 0 scene-picker strip (ItemsControl
over Scenes with staged/live border highlight); Thumbnail_MouseLeftButtonDown
moved into the control, transitioning via DataContext.TransitionToScene.

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

Controls/SceneThumbnailStrip.xaml

@@ -0,0 +1,54 @@
+<UserControl x:Class="ytLive.Controls.SceneThumbnailStrip"
+             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
+    <Border Background="#0d1117" CornerRadius="4" Margin="0,0,0,4" Padding="4">
+        <ItemsControl ItemsSource="{Binding Scenes}">
+            <ItemsControl.ItemsPanel>
+                <ItemsPanelTemplate>
+                    <UniformGrid Rows="1" Columns="5"/>
+                </ItemsPanelTemplate>
+            </ItemsControl.ItemsPanel>
+            <ItemsControl.ItemTemplate>
+                <DataTemplate>
+                    <Border CornerRadius="4" Margin="2" Padding="2"
+                            BorderThickness="2"
+                            BorderBrush="Transparent"
+                            Cursor="Hand"
+                            MouseLeftButtonDown="Thumbnail_MouseLeftButtonDown">
+                        <Border.Style>
+                            <Style TargetType="Border">
+                                <Setter Property="BorderBrush" Value="#333344"/>
+                                <Style.Triggers>
+                                    <DataTrigger Binding="{Binding IsStaged}" Value="True">
+                                        <Setter Property="BorderBrush" Value="#e94560"/>
+                                    </DataTrigger>
+                                    <DataTrigger Binding="{Binding IsLive}" Value="True">
+                                        <Setter Property="BorderBrush" Value="#22c55e"/>
+                                    </DataTrigger>
+                                </Style.Triggers>
+                            </Style>
+                        </Border.Style>
+                        <Grid>
+                            <Grid.RowDefinitions>
+                                <RowDefinition Height="*"/>
+                                <RowDefinition Height="Auto"/>
+                            </Grid.RowDefinitions>
+                            <Border Grid.Row="0" CornerRadius="3"
+                                    Background="#0a0a1a" ClipToBounds="True">
+                                <Image Source="{Binding Snapshot}"
+                                       Stretch="UniformToFill"
+                                       RenderOptions.BitmapScalingMode="LowQuality"/>
+                            </Border>
+                            <TextBlock Grid.Row="1" Foreground="#a0a0b0" FontSize="10"
+                                       HorizontalAlignment="Center"
+                                       Margin="0,2,0,0"
+                                       TextTrimming="CharacterEllipsis">
+                                <Run Text="{Binding Name, Mode=OneWay}"/><Run Text=" "/><Run Text="{Binding HotkeyLabel, Mode=OneWay}" Foreground="#6a6a7a" FontWeight="SemiBold"/>
+                            </TextBlock>
+                        </Grid>
+                    </Border>
+                </DataTemplate>
+            </ItemsControl.ItemTemplate>
+        </ItemsControl>
+    </Border>
+</UserControl>

Controls/SceneThumbnailStrip.xaml.cs

@@ -0,0 +1,21 @@
+using System.Windows;
+using System.Windows.Controls;
+using System.Windows.Input;
+using ytLive.Models;
+using ytLive.ViewModels;
+
+namespace ytLive.Controls;
+
+public partial class SceneThumbnailStrip : UserControl
+{
+    public SceneThumbnailStrip()
+    {
+        InitializeComponent();
+    }
+
+    private void Thumbnail_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
+    {
+        if (sender is Border { DataContext: Scene scene } && DataContext is MainViewModel vm)
+            vm.TransitionToScene(scene);
+    }
+}