- Replace the toast store with a notification store: levels (info/success/
warning/error/fatal), requiresInteraction, and a persisted history.
- Move toasts to the top-right, styled per level, with manual dismiss.
- Add a bell in the header opening a Notification Center (history, unread badge,
'needs attention' vs info, clear all).
- Capture network failures and 5xx responses (api layer) and render crashes
(ErrorBoundary) as notifications.
- Sound + server-sourced events + per-account settings remain for 6b.
