Takes a .docx, a text .pdf or a .txt up to 6MB, or pasted text for anything
that will not upload cleanly. The upload itself only stores the file and writes
a row - extraction runs afterwards on the server, because ptl_api is
synchronous with a five second budget and parsing a novel is not a five second
job. The page polls until the book says ready, or says why it could not be
read.
readFileB64 lives here rather than in app.js: app.js's resizeImage goes through
an <img> and a canvas, which is right for a phone photo and fatal for a .docx,
and this is the only page that uploads a document.
