feat(agent): add screen blackout feature

- Physical displays blanked via overlay
- Remote video and input keep working
- Exclude overlay from screen capture
- Add blackout API and UI toggle
- Wire flag through capture protocol

lib/screenshot/screenshot.go

@@ -16,6 +16,18 @@ import (
 
 var ErrMonitorNotFound = errors.New("monitor not found")
 
+const (
+	srcCopy    = 0x00CC0020
+	captureBlt = 0x40000000
+)
+
+var omitLayeredWindows bool
+
+// SetOmitLayered skips CAPTUREBLT so WS_EX_LAYERED overlays (blackout) stay off the bitmap.
+func SetOmitLayered(omit bool) {
+	omitLayeredWindows = omit
+}
+
 var (
 	user32                  = syscall.NewLazyDLL("user32.dll")
 	gdi32                   = syscall.NewLazyDLL("gdi32.dll")
@@ -210,8 +222,11 @@ func captureRect(r rect) (*image.RGBA, error) {
 	defer procDeleteObject.Call(bitmap)
 	old, _, _ := procSelectObject.Call(memDC, bitmap)
 	defer procSelectObject.Call(memDC, old)
-	const srccopy = 0x00CC0020 | 0x40000000 // SRCCOPY | CAPTUREBLT
-	ok, _, err := procBitBlt.Call(memDC, 0, 0, uintptr(width), uintptr(height), screenDC, uintptr(int64(r.Left)), uintptr(int64(r.Top)), srccopy)
+	blit := uintptr(srcCopy)
+	if !omitLayeredWindows {
+		blit |= captureBlt
+	}
+	ok, _, err := procBitBlt.Call(memDC, 0, 0, uintptr(width), uintptr(height), screenDC, uintptr(int64(r.Left)), uintptr(int64(r.Top)), blit)
 	if ok == 0 {
 		return nil, err
 	}