As you say, American English uses yard in much the same way that British English uses garden. The prototypical American house is situated on a plot of land in such a way as to effectively separate the plot into a front yard and a backyard. (Why the latter is a compound word and the former isn’t is a mystery for the ages.) The front yard is the more publicly facing of the two. If the plot is wide enough, there may also be a side yard (or two).
Anyway, Lee’s use ofin using the word yard here, with no other words or phrases to modify it, definitely impliesLee highly likely intended that interpretation: implying the existence—on the plot of land opposite Miss Maudie’s—of a house. If that plot is wide enough and its house is far enough down the street, then perhaps the spot where the furniture gets put could be described as being in the side yard. There’s no hard and fast boundary between side and front yards.