Simple Sync 0.3.1

Rename the whole product to Simple Sync (0.3.1)

Every user-visible name is now Simple Sync: web admin (title, header,
footer, all eight locales), plugin texts and manifest description,
server help and log messages, README, deploy files and the ZimaOS app
title. Technical identifiers stay as they are so existing installs and
the directory submission keep working: the repository name, the
ghcr.io/jirkacepelka/obsisync image, the Go module, the obsisync binary,
OBSISYNC_* variables and the ping app id.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NQQS1cszNpi5ovxnTVn8jm