fix: Improper icon sizing with respect to font size

[Yubo-Cao]

Closes N/A

Steps to test

Open JabRef and tweak font size. Now the icon in toolbar works properly under all sort of font size.

default:

13pt:

10pt:

Would greatly appreciate testing from other platforms. The screenshot above is obtained from a Windows 25H2 machine.

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.

[jabref-machine]

You committed your code on the main brach of your fork. This is a bad practice. The right way is to branch out from main, work on your patch/feature in that new branch, and then get that branch merged via the pull request (see GitHub flow).

For this pull request, this is OK. For subsequent pull requests, please start with a different branch with a proper branch name. See CONTRIBUTING.md for more details.

[trag-bot]

The comment contains a grammatical error. It should be 'to override' instead of 'is override'. Comments should be properly formatted and grammatically correct.

[Siedlerchr]

Thanks for the quick action. Will test it later today

[Siedlerchr]

Looks greaT!

[Siedlerchr]

Just one thing left I think : Filter groups search bar field has a different size maybe like the half of the other

[Siedlerchr]

merging for now