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.tsx

@@ -128,6 +128,8 @@ export default function EditorLayout() {
     toggleBulkMode, toggleSelect, clearSelection, handleBulkAction,
     stackUpdates,
     fetchImageUpdates,
+    sidebarIndicators,
+    sidebarStackUpdates,
     pinned,
     isCollapsed, toggleCollapse,
     remoteSearchLoading,
@@ -669,7 +671,7 @@ export default function EditorLayout() {
             stackLabelMap,
             stackStatuses: stackStatuses as Record<string, StackRowStatus | undefined>,
             stackCounts,
-            stackUpdates,
+            stackUpdates: sidebarStackUpdates,
             gitSourcePendingMap,
             pinnedFiles: pinned,
             isCollapsed,
@@ -697,6 +699,7 @@ export default function EditorLayout() {
           onToggleSelect={toggleSelect}
           onClearSelection={clearSelection}
           onBulkAction={handleBulkAction}
+          showUpdatesChip={sidebarIndicators}
         />
       );
 
@@ -777,6 +780,7 @@ export default function EditorLayout() {
             fleetTab={fleetTab}
             onFleetTabConsumed={() => setFleetTab(null)}
             renderEditor={renderEditor}
+            stackUpdates={stackUpdates}
           />
         </div>
       );