Fix ghost window on Windows by using proper dialog window flags

[mattezell]

On Windows 11, the UpdateCheckDialog leaves behind a transparent, non-interactive ghost window frame that persists even after the KeePassXC process has fully exited. The only way to remove this phantom window is to reboot. This has been reported and reproduced by multiple users (see #9493).

The root cause is setWindowFlags(Qt::Window) in the UpdateCheckDialog constructor. Using bare Qt::Window on a QDialog causes Windows DWM (Desktop Window Manager) to create an independently managed top-level composition surface for the dialog. If the dialog's native HWND is not cleanly destroyed through the normal DestroyWindow -> WM_NCDESTROY sequence (e.g., during process exit while the dialog exists, or during an update installation), DWM is left with an orphaned composition surface -- the title bar, borders, and shadow that DWM draws itself -- which persists as a ghost frame even after the owning process is gone.

This change replaces Qt::Window with Qt::Dialog | Qt::WindowTitleHint | Qt::WindowCloseButtonHint, which:

• Keeps the dialog properly associated with its parent window in DWM's composition tree
• Preserves the expected title bar and close button appearance
• Prevents the orphaned DWM frame by ensuring proper dialog window lifecycle management

Fixes #9493

Screenshots

N/A - This is a window flags change that prevents a ghost window artifact. The dialog's visual appearance remains the same.

Testing strategy

1. Built and ran KeePassXC on Windows 11
2. Triggered the update check dialog via Help -> Check for Updates
3. Verified the dialog appears correctly with title bar and close button
4. Closed the dialog and exited KeePassXC
5. Confirmed no ghost window frame remains after process exit

Generative AI (Claude) was used to help identify the root cause and suggest the fix.

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (change that adds functionality)
• Breaking change (causes existing functionality to change)
• Refactor (significant modification to existing code)
• Documentation (non-code change)

[Copilot]

Pull request overview

Adjusts the UpdateCheckDialog window flags to ensure it behaves as a proper dialog on Windows, preventing an orphaned “ghost” window frame from being left behind by DWM after app exit (per #9493).

Changes:

• Replaces setWindowFlags(Qt::Window) with dialog-appropriate flags (Qt::Dialog | Qt::WindowTitleHint | Qt::WindowCloseButtonHint) in UpdateCheckDialog.
• Preserves expected title bar / close button while avoiding Windows 11 ghost frame behavior.

[droidmonkey]

Looks great but fix your code format

[mattezell]

Fixed the indentation in 03b3c64. Thanks for the quick review!

[mattezell]

@droidmonkey anything that I need to action on with this?

I see the failures, but they don't seem to be related to my change - and I also notice that many other PRs have failed in what seems to be basically the same way, almost as if it's environmental in the build / test env.

[droidmonkey]

Unrelated, you are good to go here