Replaces framer-motion animation + custom SVG icons + admin-toast markup
with MUI Snackbar + MUI Alert (filled variant). useAlertState() consumption
(alerts array, removeAlert) is preserved verbatim — alert lifecycle
(timing, types, auto-dismiss) is owned by AlertContext and unchanged.
Alerts stack bottom-right with 72px vertical offset per slot.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
