Fix missing CSS for some dialogs

[HoussemNasri]

Follow up to #8939

The dialogs are:

• GlobalSearchResultDialog
• MultiMergeEntriesView

The reason behind the missing CSS is that these dialogs don't have the main window as an owner. The owner is responsible for passing the CSS stylesheets downstream, and if the owner is not set for the dialog, the CSS must be loaded explicitly. Fortunately, dialogs shown with DialogService have their owner set automatically, allowing CSS to load successfully.

Before

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[Siedlerchr]

Ah nice thanks for fixing!