Control Surface UX: pill toggle, background rename, per-scene properties

- Sliding pill toggle (green ON / red OFF) replaces checkbox
- Geometry mismatch error for custom backgrounds
- Backdrop -> Background: C# rename, SQL column rename with migration
- Source.IsBackground: new -> override (fixes WPF binding)
- Legacy 'Backdrop' display name migrated to 'Background'
- Per-scene UseDefaultBackground + CustomBackgroundPath
- Asset files renamed backdrop -> background
- StagedScene/LiveScene split, thumbnail strip, edit gating
- TransitionService (Cut/Fade/Move)
- Context menu: Capture Desktop, Refresh Desktop
- Resolution badge removed, ON-AIR status light added

Models/Scene.cs

@@ -1,6 +1,7 @@
 using System.Collections.ObjectModel;
 using System.ComponentModel;
 using System.Runtime.CompilerServices;
+using System.Windows.Media.Imaging;
 
 namespace ytLive.Models;
 
@@ -10,8 +11,44 @@ public class Scene : INotifyPropertyChanged
 
     private string _name = string.Empty;
     private bool _isHidden;
-    private bool _hasBackdrop;
+    private bool _hasBackground;
     private bool _hasSocialBar;
+    private bool _isStaged;
+    private bool _isLive;
+    private string? _customBackgroundPath;
+    private bool _useDefaultBackground = true;
+
+    public bool IsStaged
+    {
+        get => _isStaged;
+        set => Set(ref _isStaged, value);
+    }
+
+    public bool IsLive
+    {
+        get => _isLive;
+        set => Set(ref _isLive, value);
+    }
+
+    /// <summary>
+    /// User-selected background image file path (FQFN). Null means use the app default.
+    /// Stored as a reference only — the image file is not copied into the DB.
+    /// </summary>
+    public string? CustomBackgroundPath
+    {
+        get => _customBackgroundPath;
+        set => Set(ref _customBackgroundPath, value);
+    }
+
+    /// <summary>
+    /// When true, the shipped default background image is used and CustomBackgroundPath
+    /// is ignored (but preserved). Toggled by the pill-switch in the properties panel.
+    /// </summary>
+    public bool UseDefaultBackground
+    {
+        get => _useDefaultBackground;
+        set => Set(ref _useDefaultBackground, value);
+    }
 
     public string Name
     {
@@ -26,13 +63,13 @@ public class Scene : INotifyPropertyChanged
     }
 
     /// <summary>
-    /// Whether the scene has a live desktop/game backdrop layer. Enforced by
+    /// Whether the scene has a live desktop/game background layer. Enforced by
     /// policy: only the canonical Live scene ever has one (see SceneCatalog).
     /// </summary>
-    public bool HasBackdrop
+    public bool HasBackground
     {
-        get => _hasBackdrop;
-        set => Set(ref _hasBackdrop, value);
+        get => _hasBackground;
+        set => Set(ref _hasBackground, value);
     }
 
     /// <summary>
@@ -56,6 +93,18 @@ public class Scene : INotifyPropertyChanged
 
     public bool IsChatScene { get; init; }
 
+    private BitmapSource? _snapshot;
+
+    /// <summary>
+    /// Cached thumbnail snapshot of this scene for the control surface strip.
+    /// Updated by the compositor when the scene is not live-previewed.
+    /// </summary>
+    public BitmapSource? Snapshot
+    {
+        get => _snapshot;
+        set => Set(ref _snapshot, value);
+    }
+
     public event PropertyChangedEventHandler? PropertyChanged;
 
     private void Set<T>(ref T field, T value, [CallerMemberName] string? propertyName = null)