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/services/ImageUpdateService.ts

@@ -47,6 +47,7 @@ export interface ImageUpdateStatus {
     manualCooldownRemainingMs: number;
     mode: 'interval' | 'cron';
     cronExpression: string | null;
+    sidebarIndicators: boolean;
 }
 
 // ─── Compose file helpers ────────────────────────────────────────────────────
@@ -390,6 +391,13 @@ export class ImageUpdateService {
     }
 
     public getStatus(): ImageUpdateStatus {
+        let sidebarIndicators = false;
+        try {
+            const settings = DatabaseService.getInstance().getGlobalSettings();
+            sidebarIndicators = settings.image_update_sidebar_indicators === '1';
+        } catch (e) {
+            console.warn('[ImageUpdateService] Failed to read sidebar indicator setting:', e);
+        }
         return {
             checking: this.isRunning,
             intervalMinutes: Math.round(this.intervalMs / (60 * 1000)),
@@ -399,6 +407,7 @@ export class ImageUpdateService {
             manualCooldownRemainingMs: this.getManualCooldownRemainingMs(),
             mode: this.mode,
             cronExpression: this.cronExpression,
+            sidebarIndicators,
         };
     }