admin: fold dashboard sparklines into the existing cards
The trend sparklines added in #9957 lived in a separate "Cluster Trends"
row that duplicated the existing summary cards (Volumes, Files, Disk Used,
EC Shards). Remove that row and instead render each sparkline inside the
matching summary card, so every headline number shows its recent trend
without duplication. The two maintenance metrics that have no existing
card — Active Tasks and Workers — now fill the previously-empty columns of
the EC row (also with sparklines).
DashboardTrends changes from a Cards slice to named per-card sparkline
SVGs (+ current values for the two maintenance cards). Drops the now-unused
trendBytes helper (disk size keeps using the existing formatBytes).
* admin: native at-a-glance trend sparklines on the dashboard
Add a "Cluster Trends" row to the admin Dashboard with inline-SVG
sparklines for volumes, EC shards, disk used, files, active maintenance
tasks, and workers.
The data comes entirely from what the admin already holds — the cached
cluster topology and the in-process maintenance queue — sampled into a
small bounded ring buffer on the existing maintenance-metrics ticker
(~15 min of history). No Prometheus/Grafana dependency, no JS chart
library, no extra goroutine: the sparklines are self-contained SVG
rendered server-side via templ.
This gives basic trend visibility out of the box for clusters that don't
run Prometheus, and a quick glance next to the cluster controls; Grafana
remains the place for deep/historical dashboards.
* admin: cap trendBytes unit index to avoid out-of-bounds panic
A value >= 1 ZiB would push exp past the end of the units string and
panic on units[exp]; cap exp at the last unit (EiB).
Chris Lu