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/HomeDashboard.tsx

@@ -1,6 +1,7 @@
 import { useNodes } from '@/context/NodeContext';
 import type { NotificationItem } from './dashboard/types';
 import type { SectionId } from './settings/types';
+import type { StackUpdateInfo } from '@/types/imageUpdates';
 import {
   HealthStatusBar,
   ResourceGauges,
@@ -16,11 +17,12 @@ interface HomeDashboardProps {
   onOpenSettingsSection?: (section: SectionId) => void;
   notifications: NotificationItem[];
   onClearNotifications: () => void | Promise<void>;
+  stackUpdates?: Record<string, StackUpdateInfo>;
 }
 
 const NOOP = () => {};
 
-export default function HomeDashboard({ onNavigateToStack, onOpenSettingsSection, notifications, onClearNotifications }: HomeDashboardProps) {
+export default function HomeDashboard({ onNavigateToStack, onOpenSettingsSection, notifications, onClearNotifications, stackUpdates = {} }: HomeDashboardProps) {
   const { activeNode, nodes } = useNodes();
   const data = useDashboardData();
   const activeNodeName = activeNode?.name || 'Local';
@@ -49,6 +51,7 @@ export default function HomeDashboard({ onNavigateToStack, onOpenSettingsSection
         metrics={data.metrics}
         stackCpuSeries={data.stackCpuSeries}
         onNavigateToStack={onNavigateToStack ?? NOOP}
+        stackUpdates={stackUpdates}
       />
 
       <div className="grid grid-cols-1 lg:grid-cols-2 gap-4">