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/overlay_stub.go

@@ -0,0 +1,13 @@
+//go:build !windows
+
+package blackout
+
+import "errors"
+
+func running() bool { return false }
+
+func start() error {
+	return errors.New("monitor blackout is only available on Windows")
+}
+
+func stop() {}