fil3s: Improve file reader

loading entire thing into memory and then ctx.text()ing the entirety of it is Suboptimal, we can do better than that.

there's also what I think is a ctx bug where doing ctx.text with filesizes above 11200b or so (was binary searching, 10600b renders, so the actual limit is probs somewhere in the middle) crashes the entire system, at least with fil3s. for now I'm adding a filesize limit at 10240b, so this change would also allow for larger files to be shown via fil3s.