Order of authors when publishing bachelor's thesis (CompSci)
Actually, the concept of "first author" is a bit foreign to CS in most places. Authors are listed alphabetically and only contributors are listed as authors. You say the supervisor contributed to the paper and if so should be an author.

I think you are correct to refuse if it is your thesis that is being published, and I think the university will back you on it. I hope so, anyway. But for a paper "related" to the work, it is a different issue. 

But, you have a political problem. One way to handle it is not to publish a shortened version of the thesis jointly, but an extension. Publish the thesis under your own name, but acknowledge your advisor. Then get to work on an extension. 



The concept of "first author" is common in other fields, of course, but not really in CS or in math.