swarmexec v1.5.0 — Contexts tab, volume bulk-delete & prune

UI:
- New Contexts tab: list docker contexts, create/delete them, and activate
  one (u/Enter) — which restarts the UI against that cluster. The active
  context is always shown in the footer.
- Volumes: select multiple with space and delete (d), or prune every volume
  no running container mounts and no service declares (P) — each behind a
  confirm. Footer redesigned to two lines (key hints + consolidated status).
- Fix: aligned the container columns in the network detail view.

Site: a before/after demo GIF (docker exec vs swarmexec) at the top of the
README and on the landing page.

CI: the site deploy now fires via a pipeline trigger token instead of the
multi-project trigger bridge.