MichalAFerber/sheets-viewer.us ? reverse-engineered prompt

Build me a fast, mobile friendly spreadsheet viewer that runs entirely in the browser as a single HTML file.

I want to be able to drag and drop an Excel or OpenDocument workbook and browse each sheet in a clean table with clickable sheet tabs. It should also let me switch the current sheet into a plain CSV source view, with the values highlighted nicely so it is easy to inspect. This is only a viewer, not an editor, so there should be no accounts, no uploads, and no extra toolbar clutter.

Make it work offline and from a local file, and keep it private so the spreadsheet never leaves the device. If the user drops the wrong kind of file, show a polite message. Add a simple way to replace the currently open file with another one, and make the header hide itself while reading but come back when needed. If you need to check any current docs online, do that.