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.

backend/src/routes/settings.ts

@@ -32,6 +32,7 @@ const ALLOWED_SETTING_KEYS = new Set([
   'health_gate_enabled',
   'health_gate_window_seconds',
   'env_block_deploy_on_missing_required',
+  'image_update_sidebar_indicators',
 ]);
 
 // Keys whose write requires a paid license, not just an admin role.
@@ -62,6 +63,7 @@ const SettingsPatchSchema = z.object({
   health_gate_enabled: z.enum(['0', '1']),
   health_gate_window_seconds: z.coerce.number().int().min(15).max(600).transform(String),
   env_block_deploy_on_missing_required: z.enum(['0', '1']),
+  image_update_sidebar_indicators: z.enum(['0', '1']),
 }).partial();
 
 export const settingsRouter = Router();