Generate Cover has never worked: it fetched the book file using a URL
scraped from the cover preview <img> tag (so it downloaded either the
existing cover JPEG or, when no cover existed, the detail page HTML),
then handed it to foliate-js, which rejects both. Its fixed-layout path
also called view.renderer.renderPage(), a method that does not exist in
the pinned foliate fork. Every click ended in the same generic 'Cover
generation failed' toast.
The working alternative already exists server-side: the scanner's
PDF/EPUB cover extraction plus the per-book Rescan button, now that the
rasterizer renders the CropBox. Users who want a specific image can
upload one.
Delete web/src/cover-generator.ts, the modal buttons, and the dead
generateCover()/coverGenerating/fileUrl plumbing in book-detail.ts.