Hi! I think this is a most welcome addition as I recall I did sometimes in the past indeed miss a quick way to insert the current Calibre cover (the one displayed in Book Details pane) as the first PDF page. Today I'm running Calibre from latest source, and I can confirm the plugin is working great. Toolbar icon looks very distinctive in dark mode. Thanks and congratulations!
For extra PDF operations, like undoing first page insertion, merging, extracting etc. there are many open source alternatives, like PDFsam basic and such.
