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/blackout/blackout.go

@@ -0,0 +1,13 @@
+package blackout
+
+func Enabled() bool { return running() }
+
+func Set(on bool) error {
+	if on {
+		return start()
+	}
+	stop()
+	return nil
+}
+
+func Stop() { stop() }