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/capture/protocol_test.go

@@ -11,15 +11,15 @@ import (
 func TestCaptureProtocolRoundTrip(t *testing.T) {
 	t.Parallel()
 	var buf bytes.Buffer
-	if err := writeRequest(&buf, 1, "jpeg", 40); err != nil {
+	if err := writeRequest(&buf, 1, "jpeg", 40, true); err != nil {
 		t.Fatal(err)
 	}
-	mon, format, quality, err := readRequest(bufio.NewReader(bytes.NewReader(buf.Bytes())))
+	mon, format, quality, omit, err := readRequest(bufio.NewReader(bytes.NewReader(buf.Bytes())))
 	if err != nil {
 		t.Fatal(err)
 	}
-	if mon != 1 || format != "jpeg" || quality != 40 {
-		t.Fatalf("request = %d %s %d", mon, format, quality)
+	if mon != 1 || format != "jpeg" || quality != 40 || !omit {
+		t.Fatalf("request = %d %s %d omit=%v", mon, format, quality, omit)
 	}
 
 	buf.Reset()