feat(ui): Show Desktop toggle, avatar login, chat preview, backdrop seeding, border animations

- Source.ShowDesktop bool persisted in DB (schema migration + LayoutStore)
- ClearBackdropCaptureAsync clears CaptureKey + VideoImageSource for static fallback
- Top bar: red/green/pulsing status light, avatar via BitmapImage, Log In button
- ShowStartStream now requires IsConnected
- RunMockChatPreviewAsync: simple async loop, ChatPreviewEnabled property on Source
- EnsureDefaultBackdrop called on every scene switch, seeds all 5 scenes
- SeedEndingBackdrop replaces existing, SeedChatBackdrop added
- SceneCatalog: Chat scene display name corrected (layer = 'YouTube Chat')
- Left panel spacing consistency, context menu reordered
- Border animation rendering via code-behind Storyboard approach
- 3 new backdrop assets (ending-backdrop.jpg, chat-backdrop.jpg, live-backdrop.jpg)
- Build 0 warnings, 178 tests passing

Models/Source.cs

@@ -52,6 +52,9 @@ public class Source : SceneElement
     private bool _isBackdrop;
     public bool IsBackdrop { get => _isBackdrop; set => Set(ref _isBackdrop, value); }
 
+    private bool _showDesktop;
+    public bool ShowDesktop { get => _showDesktop; set => Set(ref _showDesktop, value); }
+
     private bool _isEnabled = true;
     public bool IsEnabled { get => _isEnabled; set => Set(ref _isEnabled, value); }
 
@@ -91,7 +94,9 @@ public class Source : SceneElement
 
     public ImageSource? ImageSource => _imageSource;
 
-    public override ImageSource? DisplaySource => IsLiveCapture ? VideoImageSource : _imageSource;
+    public override ImageSource? DisplaySource => IsLiveCapture
+        ? (VideoImageSource ?? _imageSource)
+        : (Type == SourceType.ChatBox ? VideoImageSource : _imageSource);
 
     public override bool IsImageSource => Type == SourceType.Image;
 
@@ -119,7 +124,34 @@ public class Source : SceneElement
     public string ChatFontColor { get => _chatFontColor; set => Set(ref _chatFontColor, value); }
 
     private string _chatBackgroundColor = "#80000000";
-    public string ChatBackgroundColor { get => _chatBackgroundColor; set => Set(ref _chatBackgroundColor, value); }
+    public string ChatBackgroundColor
+    {
+        get => _chatBackgroundColor;
+        set
+        {
+            if (Set(ref _chatBackgroundColor, value))
+                Raise(nameof(ChatBackgroundOpacity));
+        }
+    }
+
+    /// <summary>Alpha channel of ChatBackgroundColor as 0.0–1.0. Slider writes back the full hex string.</summary>
+    public double ChatBackgroundOpacity
+    {
+        get
+        {
+            var hex = _chatBackgroundColor.Trim().TrimStart('#');
+            return hex.Length == 8
+                ? byte.Parse(hex.AsSpan(0, 2), System.Globalization.NumberStyles.HexNumber) / 255.0
+                : 1.0;
+        }
+        set
+        {
+            var alpha = (byte)Math.Clamp(value * 255, 0, 255);
+            var hex = _chatBackgroundColor.Trim().TrimStart('#');
+            if (hex.Length == 8) hex = hex[2..];
+            if (hex.Length == 6) ChatBackgroundColor = $"#{alpha:X2}{hex}";
+        }
+    }
 
     private int _chatMaxMessages = 5;
     public int ChatMaxMessages
@@ -140,5 +172,22 @@ public class Source : SceneElement
     private string _chatSuperChatColor = "#eab308";
     public string ChatSuperChatColor { get => _chatSuperChatColor; set => Set(ref _chatSuperChatColor, value); }
 
+    private string _chatFontFamily = "Segoe UI";
+    public string ChatFontFamily { get => _chatFontFamily; set => Set(ref _chatFontFamily, value); }
+
+    private int _chatFadeSeconds;
+    public int ChatFadeSeconds
+    {
+        get => _chatFadeSeconds;
+        set => Set(ref _chatFadeSeconds, Math.Clamp(value, 0, 60));
+    }
+
     public bool IsChatBox => Type == SourceType.ChatBox;
+
+    private bool _chatPreviewEnabled = true;
+    public bool ChatPreviewEnabled
+    {
+        get => _chatPreviewEnabled;
+        set => Set(ref _chatPreviewEnabled, value);
+    }
 }