fix: fix not thread-safe settings singleton.

fix not thread-safe way for fetching settings singleton.
remove qApp from Settings' parent, use nullptr replace it and leak it anyway.
because we may initialize it before initializing QApplication.
in that time, qApp is nullptr.

app/settings.h

@@ -51,9 +51,7 @@ public:
     void setHiDpiScaleFactorBehavior(Qt::HighDpiScaleFactorRoundingPolicy hidpi);
 
 private:
-    Settings();
-
-    static Settings *m_settings_instance;
+    Settings(QObject *parent = nullptr);
 
     QSettings *m_qsettings;