swarmexec v1.5.1 — faster volume sizes, volume search & bulk delete

- agent: volume sizes are cached in the background (an initial scan at
  startup, refresh on volume create/destroy events, and every 10m) instead
  of a du-style scan per request, so the TUI's SIZE column fills almost
  immediately.
- ui (volumes tab): "/" search filters by name/driver/node, "a" selects all
  displayed volumes, bulk delete runs with bounded parallelism and shows a
  progress overlay.