release: cut the stable version

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FxNv1rk7jMQtb9tMBREc5B

Author: amtiYo

CHANGELOG.md

@@ -13,7 +13,7 @@ See [`docs/versioning.md`](docs/versioning.md) for what the numbers mean and
 
 Use this section for changes that are merged but not released yet.
 
-## v1.3.0-alpha.1 - 2026-09-09
+## v1.2.1 - 2026-09-09
 
 An About card, a group panel that can be closed, and the fix for a hole that let
 any group member delete Branchy's notification cards.