gui-v0.4.42

A beta channel, for a smaller group to try builds before they go out to
everyone.

• New Beta updates toggle (the flask icon next to Theme/Report-a-bug) —
  off by default. Flipping it on makes the update-checker also consider
  gui-beta-v* releases, not just gui-v* ones; flipping it off (or back
  on) re-checks immediately rather than waiting for the next launch.
• The update banner and "Update now" dialog label a beta pick as such, and
  fetch the right tag (gui-beta-v* vs gui-v*) when self-updating.
• CI (gui-build.yml) now also builds and publishes on gui-beta-v* tags,
  with a "beta build — for beta testers only" note prepended to the release
  notes.
• New beta branch in the repo — where beta-tagged work will land going
  forward, kept separate from master's stable gui-v* line.

Full Changelog: gui-v0.4.41...gui-v0.4.42