optional preview frame

src/app/AppConfig.h

@@ -2,6 +2,7 @@
 
 #include "../control/http/HttpControlServer.h"
 #include "../logging/Logger.h"
+#include "../preview/PreviewConfig.h"
 #include "../telemetry/TelemetryHealthMonitor.h"
 #include "../video/DeckLinkOutput.h"
 #include "../video/DeckLinkOutputThread.h"
@@ -29,7 +30,8 @@ struct AppConfig
 	std::string outputFrameRate = "59.94";
 	bool autoReload = true;
 	std::size_t maxTemporalHistoryFrames = 12;
-	double previewFps = 30.0;
+	bool previewEnabled = false;
+	double previewFps = kDefaultPreviewFps;
 	std::size_t warmupCompletedFrames = 4;
 	std::chrono::milliseconds warmupTimeout = std::chrono::seconds(3);
 	std::chrono::milliseconds prerollTimeout = std::chrono::seconds(3);