feat: add sidebar update indicator toggle and Stack Health badge (#1570)

* feat: add sidebar update indicator toggle and Stack Health badge

- Add image_update_sidebar_indicators setting (default off, node-scoped)
- Gate the Updates filter chip and sidebar status indicators on the setting
- Add "Update available" badge to Stack Health table (always visible)
- Extend ImageUpdateStatus with sidebarIndicators boolean
- Poll /api/image-updates/status alongside /detail in useImageUpdates
- React to SENCHO_SETTINGS_CHANGED for instant toggle propagation
- Reset sidebar state on node switch; generation-guard stale responses
- Disable toggle when status is null (loading) or field is absent (old node)
- Wire stackUpdates through ViewRouter → HomeDashboard → StackHealthTable
- Update settings registry, operator docs, and sidebar/dashboard docs

* fix: guard against stale node renders, memo drift, and cross-node error toasts

- Track owning node ID in useImageUpdates state so React never renders
  node B with node A's data before the passive effect resets (P2)
- Replace incorrect stackUpdates dependency with sidebarStackUpdates in
  chipFilteredFiles useMemo (P3)
- Guard the error toast in handleSidebarIndicatorsChange so a stale PATCH
  failure from node A does not surface while viewing node B (P3)

* fix: default sidebar update indicators to on (opt-out)

The sidebar indicators are a safe convenience that most users want.
Switching the default from off to on matches the opt-out convention
used by prune_on_update, reclaim_hero, and health_gate_enabled.

frontend/src/components/EditorLayout/ViewRouter.tsx

@@ -14,6 +14,7 @@ import type { NotificationItem } from '../dashboard/types';
 import type { ScheduleTaskPrefill } from '../ScheduledOperationsView';
 import type { MuteRuleDraft } from '@/lib/muteRules';
 import type { ActiveView } from './hooks/useViewNavigationState';
+import type { StackUpdateInfo } from '@/types/imageUpdates';
 import type { SecurityTab, FleetTab } from '@/lib/events';
 
 // Paid-tier views are loaded on demand. Their internal PaidGate /
@@ -99,6 +100,7 @@ export interface ViewRouterProps {
     // (large) editor JSX is only allocated when activeView === 'editor',
     // not on every parent render that lands on a different view.
     renderEditor: () => ReactNode;
+    stackUpdates: Record<string, StackUpdateInfo>;
 }
 
 export function ViewRouter({
@@ -128,6 +130,7 @@ export function ViewRouter({
     fleetTab,
     onFleetTabConsumed,
     renderEditor,
+    stackUpdates,
 }: ViewRouterProps): ReactNode {
     const { can } = useAuth();
     if (activeView === 'settings') {
@@ -251,6 +254,7 @@ export function ViewRouter({
             onOpenSettingsSection={onOpenSettingsSection}
             notifications={notifications}
             onClearNotifications={onClearNotifications}
+            stackUpdates={stackUpdates}
         />
     );
 }