Fix ghost window on Windows by using proper dialog window flags

Replace Qt::Window with Qt::Dialog | Qt::WindowTitleHint | Qt::WindowCloseButtonHint
in UpdateCheckDialog to prevent an orphaned DWM window frame on Windows 11.

Using bare Qt::Window on a QDialog causes Windows DWM to create an independently
managed top-level composition surface. If the dialog's native handle is not
cleanly destroyed (e.g., during process exit while the dialog exists), DWM can
leave behind a transparent, non-interactive ghost window frame that persists
even after the process is gone and can only be cleared by rebooting.

Using Qt::Dialog keeps the dialog properly associated with its parent window
in DWM's composition tree, preventing the orphaned frame.

Fixes #9493

Author: mattezell

src/gui/UpdateCheckDialog.cpp

@@ -29,7 +29,7 @@ UpdateCheckDialog::UpdateCheckDialog(QWidget* parent)
     , m_ui(new Ui::UpdateCheckDialog())
 {
     m_ui->setupUi(this);
-    setWindowFlags(Qt::Window);
+        setWindowFlags(Qt::Dialog | Qt::WindowTitleHint | Qt::WindowCloseButtonHint);
     setAttribute(Qt::WA_DeleteOnClose);
 
     m_ui->iconLabel->setPixmap(icons()->applicationIcon().pixmap(48));