Round 4 smoke feedback on /alerts:
- Bell now has consistent 12px gap from env selector and user name
(wrap env + bell in flex container inside TopBar's environment prop)
- RuleEditorWizard constrained to max-width 840px (centered) and
upgraded the page title from SectionHeader to h2 pattern used by
the list pages
- Inbox: added select-all checkbox, severity SegmentedTabs filter
(All / Critical / Warning / Info), and bulk-ack actions
(Acknowledge selected + Acknowledge all firing) alongside the
existing mark-read actions
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