fix: gate cross-node HTTP and stop-by-label on remote RBAC capability (#1509)

An older remote node ignores the forwarded actor-role header (running proxied
requests as admin) and ignores the stop-by-label stack allowlist (stopping
every label-matched stack). The control could neither detect nor prevent this
on a mixed-version fleet.

Instances now advertise a cross-node-rbac capability, and the control refuses
to act when a remote lacks it:

- HTTP proxy: a non-admin user's request is not forwarded to a remote that does
  not advertise the capability (fails closed when it cannot be determined).
  Admins are unaffected.
- Stop-by-label: a real stop bound to a confirmed stack set is not sent to a
  remote lacking the capability; the node is reported as needing an upgrade. As
  defense in depth, a node whose results name stacks outside the confirmed set
  is failed rather than rendered as a clean stop.

Separately, the stop's lock-contention path now reports every confirmed stack
as a contention failure (including one that lost its label), so a confirmed
stack is never silently dropped and the result is never empty.

frontend/src/lib/capabilities.ts

@@ -29,6 +29,7 @@ export const CAPABILITIES = [
   'env-inventory',
   'project-env-files',
   'compose-storage',
+  'cross-node-rbac',
 ] as const;
 
 export type Capability = (typeof CAPABILITIES)[number];