The easiest solution would be to show a warning when saving notes and the current revision changed since opening the dialog.

Let's keep this somewhat simple for now: Keep track of the current revision ID when opening the minutes editor, and include it in the data sent to the server. If it does not match, show a warning message that there were changes. You can then choose to overwrite (this would send the ID of the conflicting revision instead of the original one) or reload your local minutes (lose your changes).

Beyond "somewhat simple", we could also consider showing a diff, in particular for markdown, but maybe initially we can just have a "show new revision" button that opens a dialog with the rendered (in case of HTML) or raw (in case of markdown) latest revision.