Allow users to upgrade from a Stable to a Beta (manually)

Issue #1913 covers the addition of a "check for updates" button. When doing that, we should tell the users about Beta versions as well, making sure its clear that those are Beta versions (maybe a checkbox unchecked by default) and shouldn't be used in production, unless really needed.

Start with issue #1913, which covers the check-for-updates button and provides the context for this work. Define how the update flow can reveal Beta versions while clearly labeling them, with Beta selection disabled by default and an explicit production-use warning. Done means users can manually choose a Beta upgrade without obscuring the stable path.

Written by the indexing model from the issue text.