CLI: disable async crash-recovery backup in headless mode (fixes segfault)

QETProject schedules an asynchronous crash-recovery backup on construction
(writeBackup() -> QtConcurrent::run(QET::writeToFile, ..., &m_backup_file)).
In one-shot CLI mode the QETProject is destroyed as soon as the command
returns, while that background write still references its m_backup_file
member — an intermittent use-after-free segfault during teardown (~1 in 6
runs; observed on --resave and --set-titleblock).

A crash-recovery backup is meaningless for a short-lived headless command,
so add QETProject::setBackupEnabled(false), called from the CLI entry in
main(). writeBackup() then early-returns, so no background write is ever
launched. Fixes the crash for all CLI commands. See #492.

sources/main.cpp

@@ -19,6 +19,7 @@
 #include "machine_info.h"
 #include "qet.h"
 #include "qetapp.h"
+#include "qetproject.h"
 #include "singleapplication.h"
 #include "utils/macosxopenevent.h"
 #include "utils/qetsettings.h"
@@ -206,6 +207,10 @@ QGuiApplication::setHighDpiScaleFactorRoundingPolicy(QetSettings::hdpiScaleFacto
 			raw_args << QString::fromLocal8Bit(argv[i]);
 		if (CLIExport::isExportRequest(raw_args)) {
 			QApplication export_app(argc, argv);
+			// No crash-recovery backups in one-shot CLI mode: the backup write
+			// runs on a background thread referencing the project and races the
+			// process exit (intermittent segfault in QET::writeToFile).
+			QETProject::setBackupEnabled(false);
 			return CLIExport::run(export_app.arguments());
 		}
 	}