Move the Unreleased changelog under [2.15.0] — 2026-07-15, bump
versionName/versionCode to 2.15.0/21500, and sync the F-Droid
per-version changelog.
Milestone 2.15.0 (all integrated): #21#35#36#39#40#46#52.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
