Open an .ase or .aseprite file in your browser and export any frame as a PNG, with layers composited and transparency kept. Runs on your machine, with no account.
Open BitRasterFree, runs in the browser, no account needed.
PNG keeps full 8-bit alpha, so soft edges and partial transparency survive exactly. You can scale the export by an integer factor on the way out, which is a nearest-neighbour upscale rather than a blur.