* 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).