Adds a header bell component linking to /alerts/inbox with an unread-count
badge for the selected environment. Polling pauses when the tab is hidden
via TanStack Query's refetchIntervalInBackground:false (already set on
useUnreadCount); the new usePageVisible hook gives components a
re-renders-on-visibility-change signal for future defense-in-depth.
Plan-prose deviation: the plan assumed UnreadCountResponse carries a
bySeverity map for per-severity badge coloring, but the backend DTO only
exposes a scalar `count`. The bell reads `data?.count` and renders a single
var(--error) tint; a TODO references spec §13 for future per-severity work
that would require expanding the DTO.
Tests: usePageVisible toggles on visibilitychange events; NotificationBell
renders the bell with no badge at count=0 and shows "3" at count=3.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
