Adds an "Export All (JSON)" button on the Cluster Volumes page that pulls
the whole cluster's volume list from the master in one call, a superset of
volume.list. Beyond the table columns it carries garbage and fullness
ratios, modified time, compact revision, remote tiering keys, per-disk
capacity counts, EC shard sizes with file/delete counts, and a cluster-wide
duplicate-volume-id scan. Honors the active collection filter. The existing
per-page CSV export stays as "Export Page".