Bump versionName to 2.8.0 (versionCode 20800) — the merge of this to main
is what cuts the release. Move the accumulated 2.8.0 work out of
[Unreleased] into a dated CHANGELOG section and regenerate the F-Droid
per-version changelog.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
