- Add XMPP_NICKNAME config in .env for custom MUC nick (fallback to JID username)
- Add _get_muc_nick() helper: resolve nick with suffix fallback on conflict
- Add _is_my_nick() helper: compare presence nick with expected nick per room
- Handle 409 Conflict in _on_session_start: try nick alternatives (lily_, lily__)
- Handle 409 Conflict in _muc_rejoin_coro: try nick alternatives with max 3 attempts
- Stop retry when all nick variations exhausted (anti-ban: avoid infinite retry)
- Reset nick suffix counter on successful join
- Update _on_groupchat_message filter to use _is_my_nick()