refactor(notifications): extract NotificationsPage into feature module
- Add features/notifications/components/notifications-page/ with:
- useNotificationsPage (query + mark read / mark all read mutations)
- NotificationsPageHeader, NotificationsPageFilters, NotificationsPageItem
- NotificationsPageEmpty, NotificationsPageError, NotificationsPageSkeleton
- types (FilterType, NotificationTypeFilter, NOTIFICATION_TYPE_LABELS)
- Page re-exports from module; stories moved to component (Default, Loading, Error, Empty)
- MSW handler: notifications response shape aligned with notificationService (data.notifications)
Co-authored-by: Cursor <cursoragent@cursor.com>
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