fix: fix not thread-safe settings singleton.

fix not thread-safe way for fetching settings singleton.
remove qApp from Settings' parent and leak it anyway.

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;