Screen backdrop (schema v5/v6) + five-scene catalog + webcam polish: live desktop/game capture as a permanent non-deletable bottom layer (Source.IsBackdrop), auto full-screen game detection (Win32FullScreenDetector) else primary display, GraphicsCapturePicker re-designation, refcounted shared ScreenCaptureManager, Live-only backdrop by policy (HasBackdrop + v5-v6 backfill + EnforceBackdropPolicy, checkbox gone), SceneCatalog (Starting/Live/BRB/Chat/Ending) with + button re-adding missing scenes, webcam mid-session transparent-container fix (GetPreviewBitmap propagation), Chat half-screen-area cap, 'Add Webcam' always opens the picker (SwapWebcamIdentityAsync, no silent resurrect), WindowsRuntimeMarshal frame-read + 5s-throttled errors, docs updated, tests (65 passing)

Services/IFullScreenDetector.cs

@@ -0,0 +1,21 @@
+namespace ytLive.Services;
+
+/// <summary>
+/// Identifies which monitor holds a full-screen window (a game/app running
+/// borderless or exclusive fullscreen). Seam so ScreenCaptureManager is
+/// testable without Win32 interop.
+/// </summary>
+public interface IFullScreenDetector
+{
+    /// <summary>
+    /// The index of the monitor a full-screen foreground window covers, or null
+    /// when the foreground window is windowed, missing, or one of our own.
+    /// </summary>
+    int? GetForegroundFullScreenMonitorIndex();
+
+    /// <summary>Index of the primary (taskbar-hosting) monitor.</summary>
+    int PrimaryMonitorIndex();
+
+    /// <summary>All monitors in capture-key order (enumeration order).</summary>
+    IReadOnlyList<DisplayInfo> GetDisplays();
+}