feat: notification center frontend core (PR-3)
- Bell icon in icon registry (Phosphor Bell)
- NotificationProvider context with polling (15s unread / 30s list)
- useNotifications() hook with optimistic updates
- NotificationCenter component: bell + badge + dropdown panel
- NotificationItem component: severity icon, title, relative time, actions
- AppShell integration: mount in header-actions, hidden on mobile
- CSS styles: dropdown, items, unread/read states, empty state
- formatRelativeTime utility (custom, no new deps)
- 25 frontend tests (9 hook + 6 item + 10 center)
Quality gates: vitest 25 passed, tsc clean, eslint clean
alex
2bec205a30