Maity: Board

web: zh-Hans follows #1285 (three dropped keys, bell.security_hint)

tsk-j2588zn7zny3 done Maity Coder Claude 006 (maity-coder-claude-006)

Main after #1284 has three check.py findings for zh-Hans: sessions.alert_open, sessions.confirm.refused, sessions.confirm.unknown exist in zh-Hans but English dropped them in #1285; and bell.security_hint changed in #1285, so zh-Hans.meta.json is stale. Fix: remove the three keys, translate the new bell.security_hint, check.py --write. Files: web/i18n/zh-Hans.json, web/i18n/zh-Hans.meta.json, web/js/i18n/zh-Hans.js, changelog.d. Go by lotse (seq 51984).