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/openapi/spec_test.go

@@ -11,8 +11,8 @@ func TestSpec(t *testing.T) {
 	if !ok || len(paths) < 12 {
 		t.Fatalf("expected at least 12 paths, got %d", len(paths))
 	}
-	if _, ok := paths["/api/v1/mic"]; !ok {
-		t.Fatal("missing /api/v1/mic")
+	if _, ok := paths["/api/v1/blackout"]; !ok {
+		t.Fatal("missing /api/v1/blackout")
 	}
 	if _, ok := paths["/api/v1/watchdog"]; !ok {
 		t.Fatal("missing /api/v1/watchdog")