feat(docker): Docker disk is recorded over time per host, the Disk page shows each host's growth and folds images under their repository, and the overview has one disk line per host (#5304–#5306)

- #5304 "Docker disk totals recorded over time per host, with a growth chart and alertable metrics"
- #5305 "Disk page: images grouped by repository, versions folded under it"
- #5306 "Docker overview: one disk line per host linking to the Disk page"

Each `docker system df` report is now also recorded as metrics, with the host
as the resource: disk_total_bytes, its parts, and disk_reclaimable_bytes. An
alert rule can watch them. The Disk page starts with the hosts side by side,
showing how much each grew over the selected range with a line of it, then
gives each host its own section. An image's versions fold under its
repository, which is charged only for the layers its versions don't share.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

steward/static/steward.css

@@ -224,6 +224,19 @@ strong, b, th { font-weight: 500; }
 .node-tasks th, .node-tasks td { padding: 0.35rem 0.5rem; font-size: 0.82rem; }
 .node-tasks .cell-name { max-width: 12rem; }
 .node-tasks .cell-name > a { display: inline-block; max-width: 100%; vertical-align: bottom; }
+/* The Docker overview's one line per host of disk (#5306). */
+.docker-disk-line { display: flex; flex-wrap: wrap; align-items: baseline; gap: 0.35rem 1.25rem; margin: 0 0 1rem; font-size: 0.85rem; }
+.docker-disk-host { white-space: nowrap; }
+/* The Disk page (#5304, #5305): a section per host; images by repository,
+   each repository's versions folded under it. */
+.disk-host { margin: 2rem 0; }
+.disk-host-head { display: flex; flex-wrap: wrap; align-items: baseline; gap: 0.6rem; margin: 0 0 0.75rem; font-family: var(--fs-font-body); font-size: 1.05rem; font-weight: 500; }
+.disk-actions { display: flex; flex-wrap: wrap; align-items: center; gap: 0.5rem; margin-bottom: 1rem; }
+.disk-actions form { margin: 0; }
+.image-repos .cell-name { max-width: 28rem; }
+.image-repos summary { cursor: pointer; overflow: hidden; text-overflow: ellipsis; white-space: nowrap; }
+.image-repos details[open] summary { margin-bottom: 0.5rem; }
+.image-versions th, .image-versions td { padding: 0.3rem 0.5rem; font-size: 0.8rem; }
 
 /* Forms */
 .form-group { margin-bottom: 1rem; }