Fix Shift+Tab navigation in Entry Editor to focus last field of previous tab

[CXZHANG0508]

Closes #14513

This PR implements the inverse functionality of the Tab key navigation in the Entry Editor. When pressing Shift+Tab while focused on the first field of a tab, the focus now correctly jumps to the last field of the previous tab. This ensures consistent keyboard navigation flow across all tabs.

Steps to test

1. Open JabRef and open an entry in the Entry Editor.
2. Click on the first field of a tab (e.g., the first field in "Optional fields").
3. Press Shift + Tab.
4. Verify that the focus jumps to the last field of the previous tab (e.g., the last field in "Required fields").
5. Verify that pressing Shift + Tab in the middle of a tab still moves focus to the previous field within the same tab.

2025-12-12.200221.mp4

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 (if applicable)
• I added screenshots in the PR description (if change is visible to the user)
• I described the change in CHANGELOG.md in a way that is understandable for the average user (if change is visible to the user)
• [/] I checked the user 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 updating file(s) in https://github.com/JabRef/user-documentation/tree/main/en.