release: cut v2.12.0 changelog and fastlane notes

Move the accumulated [Unreleased] section under [2.12.0] — 2026-06-28 and
regenerate fastlane/.../changelogs/21200.txt from it (scripts/
sync_changelog_to_fastlane.sh), so the official F-Droid listing and the
Gitea release notes both cover the release's two changes: the past-event
display settings (agenda screen + widget, month/week dimming) and the
overlapping-event title wrapping. versionName is already 2.12.0/21200.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -5,7 +5,7 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
-## [Unreleased]
+## [2.12.0] — 2026-06-28
 
 ### Added
 - See past events your way. Two new settings change how events that have already