dteviot/WebToEpub ? reverse-engineered prompt

Build me a browser extension for Chrome and Firefox that lets me turn online novels and story pages into an EPUB file for offline reading.

I want to be able to open a supported story page, have the extension collect the chapters, clean up the text, and export a proper EPUB I can download and read in my ebook app. It should work well on common story and fanfiction sites, and if a site is not already supported, make it easy to add a new one later.

Please include a simple popup or page where I can see the detected story, choose chapters if needed, and start the export. Keep the experience straightforward and reliable, with decent error messages when a site cannot be read.

If you need to check current browser extension docs or EPUB details online, feel free to do that. Also include a clean way to test it locally and make sure the build works for both browsers.