Fix fallback window height from 786 to 768

[N1hal-tech]

(No issue associated with this small fix)

Closes

This PR fixes a small typo in the fallback window height used when JabRef corrects an out-of-bounds window position.
The previous value was 786, which is inconsistent with the intended 768 height used throughout the application’s standard fallback size (1024×768).
This ensures consistent default sizing and avoids minor UI overflow on small displays.

Steps to test

1. Remove or temporarily rename the GUI preferences file so JabRef has no stored window coordinates.
2. Start JabRef.
3. Observe that the fallback window height now correctly loads as 768 instead of 786.

Mandatory checks

• I own the copyright of the code submitted and I license it under the MIT license
• I manually tested my changes in running JabRef (always required)
• [/] I added JUnit tests for changes (not applicable – constant value change)
• [/] I added screenshots in the PR description (not applicable – no visible UI element changed)
• [/] I described the change in CHANGELOG.md in a way that is understandable for the average user (not applicable – internal minor fix)
• [/] I checked the user documentation: no updates required for this change

[github-actions]

Hey @N1hal-tech!

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[koppor]

@N1hal-tech Thank you for the fix. Could you add a CHANGELOG.md entry if possible?

[koppor]

Source code tests failing because of connection issues -- our LinkedFile is both GUI and needs remote - but we don't have a special test category for that kind of test.

[N1hal-tech]

@N1hal-tech Thank you for the fix. Could you add a CHANGELOG.md entry if possible?

Thanks! I've added the CHANGELOG entry as requested.

[koppor]

Thank you for spotting and fixing 💪

[jabref-machine]

You ticked that you modified CHANGELOG.md, but no new entry was found there.

If you made changes that are visible to the user, please add a brief description along with the issue number to the CHANGELOG.md file. If you did not, please replace the cross ([x]) by a slash ([/]) to indicate that no CHANGELOG.md entry is necessary. More details can be found in our Developer Documentation about the changelog.