swarmexec v1.8.0

New features since v1.7.0 (all backward-compatible):

- Nodes tab: cluster node info + aggregations (tasks, volumes, labels) and
  in-place node label editing.
- Placement diagnosis (inspect: D): explains why a service isn't running
  everywhere — per-node exclusion reasons (drain/down/constraint/platform) and
  per-task scheduler messages.
- :latest version check: resolves a service's real version behind the pinned
  digest, flags a newer registry image (↑), and offers a one-key update.
- Networks: encryption (ENC) column + per-network encryption in inspect,
  create-network form (attachable/encrypted/internal/IPv6/MTU/subnet/labels),
  read-only network labels.
- Volumes: create-volume form with labels (new CreateVolume agent RPC, additive)
  and read-only volume labels.
- Service editors: force-update, service spec diff, resource-limits editor,
  network-alias editing from the network detail view; multi-line env, leave-guard
  and small-terminal footer fixes.