fix(ui/alerts): bell spacing, rule editor width, inbox bulk controls

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

ui/src/components/LayoutShell.tsx

@@ -957,14 +957,14 @@ function LayoutContent() {
       <TopBar
         breadcrumb={breadcrumb}
         environment={
-          <>
+          <div style={{ display: 'flex', alignItems: 'center', gap: 12 }}>
             <EnvironmentSelector
               environments={environments}
               value={selectedEnv}
               onChange={setSelectedEnv}
             />
             <NotificationBell />
-          </>
+          </div>
         }
         user={username ? { name: username } : undefined}
         userMenuItems={userMenuItems}