Many common dialogs are displaying centered on the screen instead of the parent form

[rffrasca]

Describe the bug
Several dialogs throughout PDFKeeper are appearing in the center of the screen rather than centered relative to their owning form. This results in inconsistent UI behavior and can be disorienting when the main window is not centered.

To Reproduce
Steps to reproduce the behavior:

1. Open PDFKeeper.
2. Trigger any of the affected dialogs (e.g., open a file, confirm an action).
3. Observe that the dialog appears in the center of the screen rather than relative to the main window.

Expected behavior
Dialogs should open centered relative to the parent form and consistently set the owner window when shown.

Screenshots
If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• OS: Windows 10/11
• PDFKeeper Version: 12.0.0 and earlier