feat: add confirmed Take down stack action with optional volume removal (#1599)

* feat: add confirmed Take down stack action with optional volume removal

Expose Take down in the stack header and sidebar with a confirmation dialog
that runs compose down while keeping the stack definition on disk. Optional
volume removal is gated by node capability and stack:deploy permission, with
remote gateway preflight before proxying removeVolumes requests.

Closes #1582

* fix: reset take-down volume checkbox when dialog closes

* test: align getStackMenuVisibility assertions with showTakeDown key

getStackMenuVisibility now returns a fifth lifecycle flag, showTakeDown,
but three exhaustive toEqual assertions still listed only the prior four
keys and failed. Add the expected showTakeDown value to each: true for
the partial and exited running-stack cases, false for the self stack.

* test: cover Take down visibility for running non-self stacks

The getStackMenuVisibility assertions exercised the partial and exited
branches and the self-stack guard, but not the raw === 'running' literal
that drives showTakeDown for a normal running stack. Add a case so a
regression dropping 'running' from that check is caught.

* fix: drop Take down from header overflow and wire activity shortcut

Remove duplicate Take down from More actions.

Keep inline button when running, sidebar menu, and Cmd+ArrowDown.

Record stack_taken_down in activity on successful POST /down.

backend/src/services/NotificationService.ts

@@ -19,6 +19,7 @@ export type NotificationCategory =
     | 'stack_started'
     | 'stack_stopped'
     | 'stack_restarted'
+    | 'stack_taken_down'
     | 'image_update_available'
     | 'image_update_applied'
     | 'autoheal_triggered'
@@ -43,7 +44,7 @@ export type NotificationCategory =
 
 export const ALL_NOTIFICATION_CATEGORIES: readonly NotificationCategory[] = [
     'deploy_success', 'deploy_failure', 'stack_started', 'stack_stopped',
-    'stack_restarted', 'image_update_available', 'image_update_applied',
+    'stack_restarted', 'stack_taken_down', 'image_update_available', 'image_update_applied',
     'autoheal_triggered', 'monitor_alert', 'scan_finding',
     'blueprint_deployed', 'blueprint_deployment_failed',
     'blueprint_drift_detected', 'blueprint_drift_correction_failed',