feat(fleet): add Fleet Actions tab for cross-node bulk operations (#963)

* feat(fleet): add Fleet Actions tab for cross-node bulk operations

Introduces a new "Actions" sub-tab in Fleet view with two Skipper+ cards
that fill gaps in the existing surface:

- Stop fleet by label: matches a label name across every node and stops
  every stack assigned to it, reporting per-node and per-stack results.
- Bulk label assign: applies the same label set to many stacks on one
  node in a single round trip.

Other bulk operations stay in their existing homes (sidebar bulk mode,
Schedules, NodeUpdatesSheet) to avoid duplicate surfaces.

Backend:
- POST /api/fleet/labels/fleet-stop (gateway-orchestrated, multi-node)
- POST /api/fleet-actions/labels/bulk-assign (per-node, capped at 1000)
- Tightens /api/fleet proxy-exempt prefix to /api/fleet/ so
  /api/fleet-actions/* is routed through the proxy for per-node calls.
- Exports activeBulkActions from labels.ts so fleet-stop and label-action
  share the per-node lock and cannot double-stop the same containers.
- Extracts containerActionForStack helper from stacks.ts for reuse.

* chore(fleet): rename Actions tab to Fleet Actions and reorder Fleet sub-tabs

- Tab label "Actions" -> "Fleet Actions" so the surface is unambiguous
  alongside Schedules and the sidebar bulk bar.
- Reorder Fleet sub-tabs as Overview / Snapshots / Status | Deployments /
  Traffic / Fleet Actions, with the separator after Status.
- Rename "Traffic · Routing" -> "Traffic" and update Sencho Mesh docs to
  match the shorter label.
- Update Fleet Actions docs to the new tab name and placement.

backend/src/routes/labels.ts

@@ -15,7 +15,11 @@ import { getErrorMessage, isSqliteUniqueViolation } from '../utils/errors';
 import { parseIntParam } from '../utils/parseIntParam';
 import { sanitizeForLog } from '../utils/safeLog';
 
-const activeBulkActions = new Set<string>();
+// Module-scope lock shared by `POST /api/labels/:id/action` and the fleet-wide
+// bulk endpoints in `routes/fleet.ts`. Keyed by `${nodeId}` so concurrent bulk
+// actions targeting the same node serialize and a fleet-stop cannot race a
+// per-label action on the same containers.
+export const activeBulkActions = new Set<string>();
 
 export const labelsRouter = Router();