Layers panel and scene strip: scene layers header with icons, per-type layer icons, PROGRAM badge and selected/live orange borders, scene name and hotkey overlays, Starting as default scene

- LeftPanel: SCENE LAYERS header with layer count; per-type blue icons; properties header gear; layers list min/max height; properties placeholder; list and properties indent
- SceneThumbnailStrip: names/hotkeys over the tile; PROGRAM badge with blinking red dot on the selected tile; orange border on selected and live tiles; fixed the element border override that hid the style border; reflag tiles on load
- LiveScene is now set on go-live, record start, and live transitions (it was never assigned, so IsLive never fired)
- Startup stages the scene named Starting
- DESIGN.md: color roles (red = YouTube/on-air/record, orange #ff8c3a = attention, amber = game audio/Super Chat)
- Staged live tile uses background art instead of the green LIVE placeholder

Not pushed.

Models/Source.cs

@@ -73,6 +73,7 @@ public class Source : SceneElement
                 Raise(nameof(IsLiveCapture));
                 Raise(nameof(DisplaySource));
                 Raise(nameof(IsWebSource));
+                Raise(nameof(IsTextOverlay));
             }
         }
     }
@@ -85,6 +86,7 @@ public class Source : SceneElement
         ? ElementKind.Dynamic : ElementKind.Static;
 
     public override bool IsWebSource => Type == SourceType.WebSource;
+    public override bool IsTextOverlay => Type == SourceType.TextOverlay;
 
     /// <summary>The permanent scene background (live desktop/game capture). Never removable/reorderable.</summary>
     private bool _isBackground;
@@ -110,7 +112,7 @@ public class Source : SceneElement
     private string? _captureKey;
     public string? CaptureKey { get => _captureKey; set => Set(ref _captureKey, value); }
 
-    public bool IsLiveCapture => Type is SourceType.DisplayCapture or SourceType.WindowCapture;
+    public override bool IsLiveCapture => Type is SourceType.DisplayCapture or SourceType.WindowCapture;
 
     // Image (asset stored in the layout database)
     private string? _assetId;
@@ -190,7 +192,7 @@ public class Source : SceneElement
         set => Set(ref _mediaPlaybackState, value);
     }
 
-    public bool IsMediaSource => Type == SourceType.MediaSource;
+    public override bool IsMediaSource => Type == SourceType.MediaSource;
 
     // Alert video config (TASK 47): the shipped default clip lives in the Asset
     // table (seeded like image assets); the creator's replacement is a FILE PATH