Closes #14083

Converted fixed-value ComboBoxes in the Preferences dialog & Web Search to SearchableComboBoxes for improved usability.
Updated sections include General, Web Search, Entry Types, and External Programs.

Steps to test

In Web Search, click the Service dropdown and use keyboard input to quickly jump to and cycle among matching services.

In Preferences → General, open the Language dropdown and filter languages by typing.

In Preferences → Entry Types, add or edit a field and observe that the Field selection dropdown now live-filters as you type.

In Preferences → External Programs → Push Applications → Application to push entries to, use the dropdown to confirm it filters results dynamically as you type.

Known Observations

While testing Preferences → General → Language, I noticed that the SearchableComboBox only matches exact text.
For example, searching for “espanol” does not bring up “Español,” and “filipino” does not match “Tagalog/Filipino.”
This affects certain languages (e.g., Español, Deutsch, Suomi, Nederlands, Tagalog/Filipino, Français) where accents or alternate spellings prevent matches.

In Preferences → AI → Chat model dropdown, users can type inside the field, but it does not filter or search the choices as expected. Instead, it allows input but does not match or autocomplete available models. This may cause confusion, as it appears to be editable but does not work as a searchable/filterable dropdown.

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.