feat(scheduler): consistent action targeting in Scheduled Operations (#1431)

Give every scheduled action an explicit, predictable target model
(Action then Node then Stack then Options then Schedule):

- System Prune now exposes a Node picker and requires a node, so it can
  no longer run silently on the default node.
- Vulnerability Scan and System Prune list local nodes only; both run on
  the hub-local Docker daemon and reject remote nodes on the backend.
- Restart Stack service discovery loads services from the selected node
  via fetchForNode instead of the active or local node.
- Fleet Snapshot shows a read-only "Scope: Entire fleet" summary.

Backend gains a shared local-node guard and prune node validation on
create and update, plus an executor-level remote-node guard, so the
frontend and backend validation now agree for every action.

frontend/src/lib/__tests__/scheduledActions.test.ts

@@ -38,6 +38,11 @@ describe('scheduledActions registry', () => {
     }
   });
 
+  it('marks scan and prune as local-node actions', () => {
+    expect(getActionById('scan')).toMatchObject({ requiresNode: true, nodeScope: 'local' });
+    expect(getActionById('prune')).toMatchObject({ requiresNode: true, nodeScope: 'local' });
+  });
+
   it('getActionById resolves a known id and returns undefined otherwise', () => {
     expect(getActionById('restart')?.label).toBe('Restart Stack');
     expect(getActionById('nope')).toBeUndefined();