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.

docs/reference/settings.mdx

@@ -465,6 +465,16 @@ Configure how often this node polls container registries to detect available ima
 
 The section footer shows the last-checked timestamp and when the next check is scheduled.
 
+### Sidebar
+
+| Setting | Default | Description |
+|---------|---------|-------------|
+| **Show update status in sidebar** | On | When on, the sidebar shows a pulsing dot on stacks with an available update, a warning icon when a check fails, and an Updates filter chip. The Stack Health table on the home page always shows update status regardless of this setting. Notifications are unaffected. |
+
+<Note>
+  Nodes running older versions of Sencho do not expose this setting. Upgrade the node to enable the toggle.
+</Note>
+
 ---
 
 ## Webhooks