v2.3.1 — In-app version display

Fixed

• In-app version display (#390). 2.3.0 reported itself as "2.2.1" in the header, the "update available" banner, and telemetry — the __version__ constant wasn't bumped alongside pyproject (the two were maintained separately and the release procedure only mentioned the latter). Bumped, plus a CI test that now fails if they ever drift again, and a release-procedure note. No functional change.