Your second assumption is right. 'Figured' means thought, imagined, guessed, worked out, presumed and so on. 'Could use' means need, in the sense that it would be welcome. For example, if you have been trying to do a difficult job on your own, you might say 'I could use some help'. 'One' refers back to 'a lunch break'. So the sentence as a whole means 'I guessed that you need a break'. Not necessarily food - just a break. By the way, could I give you some advice? Don't write 'wanna' unless you are writing or quoting subtitles, cartoon dialogues or popular song lyrics. Unlike 'don't' 'can't' 'I'm' and other contractions, 'wanna' is not an accepted part of the written language. Nor is 'I' written with a small 'i' - the pronoun is always written with a capital 'I'. These sloppy habits make the writer look lazy, rude and uneducated. Nobody will take you seriously if you write like this.

"use" in this context means "benefit" from. In the context, the person being spoken to may need a lunch break though the reason is not given - perhaps he or she is looking tired or hungry.