Closes #14237

I fixed an issue where keyboard shortcuts could not be edited in:
Preferences → Keyboard shortcuts.

The KeyEvent was being consumed by global application shortcuts before reaching
the shortcut editor in the KeyBindingsTab. As a result, pressing keys did not
update the selected shortcut, and changes were not persisted.

This PR adds:

• A key event filter in KeyBindingsTab to ensure the TreeTableView receives key presses.
• Logic to forward the event to the ViewModel (setNewBindingForCurrent).
• Correct persistence of edited shortcuts by syncing the repository into GuiPreferences.
• An updated JUnit test verifying the binding update behavior.

Steps to test

1. Open JabRef → Preferences → Keyboard shortcuts.
2. Select any shortcut entry (e.g., “Close library”).
3. Press a new key combination (e.g., Ctrl+Shift+L).
4. Click Save.
5. Restart JabRef.
6. Verify that:
   • The new shortcut appears in the list.
   • The shortcut works in the application (e.g., Ctrl+Shift+L closes the library).

Screenshots visible changes are not required since UI remains the same.

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 (not applicable — UI did not change)
• [/] I described the change in CHANGELOG.md (not applicable — minor behavior fix)
• [/] I checked the user documentation: no updates required since UI workflow remains identical