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/SceneElement.cs

@@ -84,6 +84,9 @@ public abstract class SceneElement : INotifyPropertyChanged
 
     /// <summary>True only for AlertBox-type sources. Gates stream alerts properties panel.</summary>
     public virtual bool IsAlertBox => false;
+    public virtual bool IsTextOverlay => false;
+    public virtual bool IsMediaSource => false;
+    public virtual bool IsLiveCapture => false;
     public virtual bool IsBackground { get; set; }
 
     private bool _useDefaultBackground = true;